Close #20275: Optimize BaseEventLoop._run_once()

Logger.log() is "slow", logger.isEnabledFor() is faster and the logger is
disabled in most cases. A microbenchmark executing 100,000 dummy tasks is 22%
faster with this change.

Lib/asyncio/base_events.py

@@ -610,6 +610,7 @@ class BaseEventLoop(events.AbstractEventLoop):
                 timeout = min(timeout, deadline)
 
         # TODO: Instrumentation only in debug mode?
+        if logger.isEnabledFor(logging.INFO):
             t0 = self.time()
             event_list = self._selector.select(timeout)
             t1 = self.time()
@@ -619,6 +620,8 @@ class BaseEventLoop(events.AbstractEventLoop):
             else:
                 level = logging.DEBUG
             logger.log(level, 'poll%s took %.3f seconds', argstr, t1-t0)
+        else:
+            event_list = self._selector.select(timeout)
         self._process_events(event_list)
 
         # Handle 'later' callbacks that are ready.