diff --git a/changelog.d/6126.feature b/changelog.d/6126.feature
new file mode 100644
index 0000000000..1207ba6206
--- /dev/null
+++ b/changelog.d/6126.feature
@@ -0,0 +1 @@
+Group events into larger federation transactions at times of high traffic.
diff --git a/synapse/federation/sender/__init__.py b/synapse/federation/sender/__init__.py
index d46f4aaeb1..497485fac2 100644
--- a/synapse/federation/sender/__init__.py
+++ b/synapse/federation/sender/__init__.py
@@ -152,9 +152,24 @@ class FederationSender(object):
 
     @defer.inlineCallbacks
     def _process_event_queue_loop(self):
+        loop_start_time = self.clock.time_msec()
         try:
             self._is_processing = True
             while True:
+                # if we've been going around this loop for a long time without
+                # catching up, deprioritise transaction transmission. This should mean
+                # that events get batched into fewer transactions, which is more
+                # efficient, and hence give us a chance to catch up
+                if (
+                    self.clock.time_msec() - loop_start_time > 60 * 1000
+                    and not self._transaction_manager.deprioritise_transmission
+                ):
+                    logger.warning(
+                        "Event processing loop is getting behind: deprioritising "
+                        "transaction transmission"
+                    )
+                    self._transaction_manager.deprioritise_transmission = True
+
                 last_token = yield self.store.get_federation_out_pos("events")
                 next_token, events = yield self.store.get_all_new_events_stream(
                     last_token, self._last_poked_id, limit=100
@@ -251,6 +266,9 @@ class FederationSender(object):
 
         finally:
             self._is_processing = False
+            if self._transaction_manager.deprioritise_transmission:
+                logger.info("Event queue caught up: re-prioritising transmission")
+                self._transaction_manager.deprioritise_transmission = False
 
     def _send_pdu(self, pdu, destinations):
         # We loop through all destinations to see whether we already have
diff --git a/synapse/federation/sender/per_destination_queue.py b/synapse/federation/sender/per_destination_queue.py
index fad980b893..b890aaf840 100644
--- a/synapse/federation/sender/per_destination_queue.py
+++ b/synapse/federation/sender/per_destination_queue.py
@@ -189,6 +189,11 @@ class PerDestinationQueue(object):
 
             pending_pdus = []
             while True:
+                if self._transaction_manager.deprioritise_transmission:
+                    # if the event-processing loop has got behind, sleep to give it
+                    # a chance to catch up
+                    yield self._clock.sleep(2)
+
                 # We have to keep 2 free slots for presence and rr_edus
                 limit = MAX_EDUS_PER_TRANSACTION - 2
 
diff --git a/synapse/federation/sender/transaction_manager.py b/synapse/federation/sender/transaction_manager.py
index 5b6c79c51a..69679dbf65 100644
--- a/synapse/federation/sender/transaction_manager.py
+++ b/synapse/federation/sender/transaction_manager.py
@@ -49,6 +49,10 @@ class TransactionManager(object):
         # HACK to get unique tx id
         self._next_txn_id = int(self.clock.time_msec())
 
+        # the federation sender sometimes sets this to delay transaction transmission,
+        # if the sender gets behind.
+        self.deprioritise_transmission = False
+
     @measure_func("_send_new_transaction")
     @defer.inlineCallbacks
     def send_new_transaction(self, destination, pending_pdus, pending_edus):