`@gen_cluster` has become 1s slower; CI takes 50% longer

Since #6603, a test decorated with `@gen_cluster`:
```python
@gen_cluster()
async def test1(s, a, b):
    pass
```
has increased in runtime on my host from 220ms to 1220ms.

With the same PR, the CI test suites (ci1) have increased:
Ubuntu: 19m23s -> 34m18s
Windows: 23m36s -> 48m55s
MacOSX: 32m15s -> 46m39s

CC @graingert 