diff --git a/Lib/asyncio/proactor_events.py b/Lib/asyncio/proactor_events.py
index ff6d08f78ee..9636c6b4d28 100644
--- a/Lib/asyncio/proactor_events.py
+++ b/Lib/asyncio/proactor_events.py
@@ -459,6 +459,7 @@ class _ProactorDatagramTransport(_ProactorBasePipeTransport,
                  waiter=None, extra=None):
         self._address = address
         self._empty_waiter = None
+        self._buffer_size = 0
         # We don't need to call _protocol.connection_made() since our base
         # constructor does it for us.
         super().__init__(loop, sock, protocol, waiter=waiter, extra=extra)
@@ -471,7 +472,7 @@ class _ProactorDatagramTransport(_ProactorBasePipeTransport,
         _set_socket_extra(self, sock)
 
     def get_write_buffer_size(self):
-        return sum(len(data) for data, _ in self._buffer)
+        return self._buffer_size
 
     def abort(self):
         self._force_close(None)
@@ -496,6 +497,7 @@ class _ProactorDatagramTransport(_ProactorBasePipeTransport,
 
         # Ensure that what we buffer is immutable.
         self._buffer.append((bytes(data), addr))
+        self._buffer_size += len(data)
 
         if self._write_fut is None:
             # No current write operations are active, kick one off
@@ -522,6 +524,7 @@ class _ProactorDatagramTransport(_ProactorBasePipeTransport,
                 return
 
             data, addr = self._buffer.popleft()
+            self._buffer_size -= len(data)
             if self._address is not None:
                 self._write_fut = self._loop._proactor.send(self._sock,
                                                             data)
diff --git a/Lib/asyncio/selector_events.py b/Lib/asyncio/selector_events.py
index e99a50395e7..c9bbe2ac014 100644
--- a/Lib/asyncio/selector_events.py
+++ b/Lib/asyncio/selector_events.py
@@ -1131,6 +1131,7 @@ class _SelectorDatagramTransport(_SelectorTransport):
                  waiter=None, extra=None):
         super().__init__(loop, sock, protocol, extra)
         self._address = address
+        self._buffer_size = 0
         self._loop.call_soon(self._protocol.connection_made, self)
         # only start reading when connection_made() has been called
         self._loop.call_soon(self._add_reader,
@@ -1141,7 +1142,7 @@ class _SelectorDatagramTransport(_SelectorTransport):
                                  waiter, None)
 
     def get_write_buffer_size(self):
-        return sum(len(data) for data, _ in self._buffer)
+        return self._buffer_size
 
     def _read_ready(self):
         if self._conn_lost:
@@ -1200,11 +1201,13 @@ class _SelectorDatagramTransport(_SelectorTransport):
 
         # Ensure that what we buffer is immutable.
         self._buffer.append((bytes(data), addr))
+        self._buffer_size += len(data)
         self._maybe_pause_protocol()
 
     def _sendto_ready(self):
         while self._buffer:
             data, addr = self._buffer.popleft()
+            self._buffer_size -= len(data)
             try:
                 if self._extra['peername']:
                     self._sock.send(data)
@@ -1212,6 +1215,7 @@ class _SelectorDatagramTransport(_SelectorTransport):
                     self._sock.sendto(data, addr)
             except (BlockingIOError, InterruptedError):
                 self._buffer.appendleft((data, addr))  # Try again later.
+                self._buffer_size += len(data)
                 break
             except OSError as exc:
                 self._protocol.error_received(exc)
diff --git a/Misc/NEWS.d/next/Library/2022-04-15-19-34-02.gh-issue-91487.2aqguF.rst b/Misc/NEWS.d/next/Library/2022-04-15-19-34-02.gh-issue-91487.2aqguF.rst
new file mode 100644
index 00000000000..d3c41a7e4cb
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2022-04-15-19-34-02.gh-issue-91487.2aqguF.rst
@@ -0,0 +1 @@
+Optimize asyncio UDP speed, over 100 times faster when transferring a large file.