diff --git a/substrate/core/consensus/common/src/import_queue/basic_queue.rs b/substrate/core/consensus/common/src/import_queue/basic_queue.rs
index 082006ae5d..d94c3a383c 100644
--- a/substrate/core/consensus/common/src/import_queue/basic_queue.rs
+++ b/substrate/core/consensus/common/src/import_queue/basic_queue.rs
@@ -14,12 +14,12 @@
 // You should have received a copy of the GNU General Public License
 // along with Substrate.  If not, see <http://www.gnu.org/licenses/>.
 
-use std::{pin::Pin, sync::Arc};
+use std::{mem, pin::Pin, sync::Arc, time::Duration};
 use futures::{prelude::*, channel::mpsc, task::SpawnExt as _, task::Context, task::Poll};
+use futures_timer::Delay;
 use runtime_primitives::{Justification, traits::{Block as BlockT, Header as HeaderT, NumberFor}};
 
-use crate::error::Error as ConsensusError;
-use crate::block_import::{BlockImport, BlockOrigin};
+use crate::block_import::BlockOrigin;
 use crate::import_queue::{
 	BlockImportResult, BlockImportError, Verifier, BoxBlockImport, BoxFinalityProofImport,
 	BoxJustificationImport, ImportQueue, Link, Origin,
@@ -136,10 +136,10 @@ enum ToWorkerMsg<B: BlockT> {
 
 struct BlockImportWorker<B: BlockT, V: Verifier<B>> {
 	result_sender: BufferedLinkSender<B>,
-	block_import: BoxBlockImport<B>,
 	justification_import: Option<BoxJustificationImport<B>>,
 	finality_proof_import: Option<BoxFinalityProofImport<B>>,
 	verifier: Arc<V>,
+	delay_between_blocks: Duration,
 }
 
 impl<B: BlockT, V: 'static + Verifier<B>> BlockImportWorker<B, V> {
@@ -156,24 +156,55 @@ impl<B: BlockT, V: 'static + Verifier<B>> BlockImportWorker<B, V> {
 			result_sender,
 			verifier,
 			justification_import,
-			block_import,
 			finality_proof_import,
+			delay_between_blocks: Duration::new(0, 0),
 		};
 
+		// Let's initialize `justification_import` and `finality_proof_import`.
 		if let Some(justification_import) = worker.justification_import.as_mut() {
 			for (hash, number) in justification_import.on_start() {
 				worker.result_sender.request_justification(&hash, number);
 			}
 		}
-
 		if let Some(finality_proof_import) = worker.finality_proof_import.as_mut() {
 			for (hash, number) in finality_proof_import.on_start() {
 				worker.result_sender.request_finality_proof(&hash, number);
 			}
 		}
 
+		// The future below has two possible states:
+		//
+		// - Currently importing many blocks, in which case `importing` is `Some` and contains a
+		//   `Future`, and `block_import` is `None`.
+		// - Something else, in which case `block_import` is `Some` and `importing` is None.
+		//
+		let mut block_import = Some(block_import);
+		let mut importing = None;
+
 		let future = futures::future::poll_fn(move |cx| {
 			loop {
+				// If the results sender is closed, that means that the import queue is shutting
+				// down and we should end this future.
+				if worker.result_sender.is_closed() {
+					return Poll::Ready(())
+				}
+
+				// If we are in the process of importing a bunch of block, let's resume this
+				// process before doing anything more.
+				if let Some(imp_fut) = importing.as_mut() {
+					match Future::poll(Pin::new(imp_fut), cx) {
+						Poll::Pending => return Poll::Pending,
+						Poll::Ready(bi) => {
+							block_import = Some(bi);
+							importing = None;
+						},
+					}
+				}
+
+				debug_assert!(importing.is_none());
+				debug_assert!(block_import.is_some());
+
+				// Grab the next action request sent to the import queue.
 				let msg = match Stream::poll_next(Pin::new(&mut port), cx) {
 					Poll::Ready(Some(msg)) => msg,
 					Poll::Ready(None) => return Poll::Ready(()),
@@ -182,7 +213,10 @@ impl<B: BlockT, V: 'static + Verifier<B>> BlockImportWorker<B, V> {
 
 				match msg {
 					ToWorkerMsg::ImportBlocks(origin, blocks) => {
-						worker.import_a_batch_of_blocks(origin, blocks);
+						// On blocks import request, we merely *start* the process and store
+						// a `Future` into `importing`.
+						let bi = block_import.take().expect("block_import is always Some; qed");
+						importing = Some(worker.import_a_batch_of_blocks(bi, origin, blocks));
 					},
 					ToWorkerMsg::ImportFinalityProof(who, hash, number, proof) => {
 						worker.import_finality_proof(who, hash, number, proof);
@@ -197,17 +231,24 @@ impl<B: BlockT, V: 'static + Verifier<B>> BlockImportWorker<B, V> {
 		(future, sender)
 	}
 
-	fn import_a_batch_of_blocks(&mut self, origin: BlockOrigin, blocks: Vec<IncomingBlock<B>>) {
-		let result_sender = &self.result_sender;
-		let (imported, count, results) = import_many_blocks(
-			&mut *self.block_import,
-			origin,
-			blocks,
-			self.verifier.clone(),
-			|| !result_sender.is_closed(),
-		);
+	/// Returns a `Future` that imports the given blocks and sends the results on
+	/// `self.result_sender`.
+	///
+	/// For lifetime reasons, the `BlockImport` implementation must be passed by value, and is
+	/// yielded back in the output once the import is finished.
+	fn import_a_batch_of_blocks(
+		&mut self,
+		block_import: BoxBlockImport<B>,
+		origin: BlockOrigin,
+		blocks: Vec<IncomingBlock<B>>
+	) -> impl Future<Output = BoxBlockImport<B>> {
+		let mut result_sender = self.result_sender.clone();
 
-		self.result_sender.blocks_processed(imported, count, results);
+		import_many_blocks(block_import, origin, blocks, self.verifier.clone(), self.delay_between_blocks)
+			.then(move |(imported, count, results, block_import)| {
+				result_sender.blocks_processed(imported, count, results);
+				future::ready(block_import)
+			})
 	}
 
 	fn import_finality_proof(&mut self, who: Origin, hash: B::Hash, number: NumberFor<B>, finality_proof: Vec<u8>) {
@@ -257,20 +298,22 @@ impl<B: BlockT, V: 'static + Verifier<B>> BlockImportWorker<B, V> {
 
 /// Import several blocks at once, returning import result for each block.
 ///
-/// The `keep_going` closure will be called regularly. If it returns false, then the function will
-/// end prematurely.
+/// For lifetime reasons, the `BlockImport` implementation must be passed by value, and is yielded
+/// back in the output once the import is finished.
+///
+/// The returned `Future` yields at every imported block, which makes the execution more
+/// fine-grained and making it possible to interrupt the process.
 fn import_many_blocks<B: BlockT, V: Verifier<B>>(
-	import_handle: &mut dyn BlockImport<B, Error = ConsensusError>,
+	import_handle: BoxBlockImport<B>,
 	blocks_origin: BlockOrigin,
 	blocks: Vec<IncomingBlock<B>>,
 	verifier: Arc<V>,
-	keep_going: impl Fn() -> bool,
-) -> (usize, usize, Vec<(
+	delay_between_blocks: Duration,
+) -> impl Future<Output = (usize, usize, Vec<(
 	Result<BlockImportResult<NumberFor<B>>, BlockImportError>,
 	B::Hash,
-)>) {
+)>, BoxBlockImport<B>)> {
 	let count = blocks.len();
-	let mut imported = 0;
 
 	let blocks_range = match (
 		blocks.first().and_then(|b| b.header.as_ref().map(|h| h.number())),
@@ -283,38 +326,72 @@ fn import_many_blocks<B: BlockT, V: Verifier<B>>(
 
 	trace!(target: "sync", "Starting import of {} blocks {}", count, blocks_range);
 
+	let mut imported = 0;
 	let mut results = vec![];
-
 	let mut has_error = false;
+	let mut blocks = blocks.into_iter();
+	let mut import_handle = Some(import_handle);
+	let mut waiting = None;
 
 	// Blocks in the response/drain should be in ascending order.
-	for block in blocks {
-		if !keep_going() {
-			// Setting `has_error` to true cancels the rest of the import.
-			has_error = true;
+
+	future::poll_fn(move |cx| {
+		// Handle the optional timer that makes us wait before the next import.
+		if let Some(waiting) = &mut waiting {
+			match Future::poll(Pin::new(waiting), cx) {
+				Poll::Ready(_) => {},
+				Poll::Pending => return Poll::Pending,
+			}
 		}
+		waiting = None;
+
+		// Is there any block left to import?
+		let block = match blocks.next() {
+			Some(b) => b,
+			None => {
+				// No block left to import, success!
+				let import_handle = import_handle.take()
+					.expect("Future polled again after it has finished");
+				let results = mem::replace(&mut results, Vec::new());
+				return Poll::Ready((imported, count, results, import_handle));
+			},
+		};
+
+		// We extract the content of `import_handle` only when the future ends, therefore
+		// `import_handle` is always `Some` here. It is illegal to poll a `Future` again after it
+		// has ended.
+		let import_handle = import_handle.as_mut()
+			.expect("Future polled again after it has finished");
 
 		let block_number = block.header.as_ref().map(|h| h.number().clone());
 		let block_hash = block.hash;
 		let import_result = if has_error {
 			Err(BlockImportError::Cancelled)
 		} else {
+			// The actual import.
 			import_single_block(
-				import_handle,
+				&mut **import_handle,
 				blocks_origin.clone(),
 				block,
 				verifier.clone(),
 			)
 		};
-		let was_ok = import_result.is_ok();
-		if was_ok {
+
+		if import_result.is_ok() {
 			trace!(target: "sync", "Block imported successfully {:?} ({})", block_number, block_hash);
 			imported += 1;
 		} else {
 			has_error = true;
 		}
-		results.push((import_result, block_hash));
-	}
 
-	(imported, count, results)
+		results.push((import_result, block_hash));
+
+		// Notifies the current task again so that we re-execute this closure again for the next
+		// block.
+		if delay_between_blocks != Duration::new(0, 0) {
+			waiting = Some(Delay::new(delay_between_blocks));
+		}
+		cx.waker().wake_by_ref();
+		Poll::Pending
+	})
 }
diff --git a/substrate/core/consensus/common/src/import_queue/buffered_link.rs b/substrate/core/consensus/common/src/import_queue/buffered_link.rs
index ffd08e690a..90921bd1ea 100644
--- a/substrate/core/consensus/common/src/import_queue/buffered_link.rs
+++ b/substrate/core/consensus/common/src/import_queue/buffered_link.rs
@@ -66,6 +66,14 @@ impl<B: BlockT> BufferedLinkSender<B> {
 	}
 }
 
+impl<B: BlockT> Clone for BufferedLinkSender<B> {
+	fn clone(&self) -> Self {
+		BufferedLinkSender {
+			tx: self.tx.clone(),
+		}
+	}
+}
+
 /// Internal buffered message.
 enum BlockImportWorkerMsg<B: BlockT> {
 	BlocksProcessed(usize, usize, Vec<(Result<BlockImportResult<NumberFor<B>>, BlockImportError>, B::Hash)>),