Long time to lower n-window

[MetRonnie]

Description

Observed a large workflow take ~ 1 minute to drop from N=3 to N=1. This was a combination of maxing out 1 CPU core on the scheduler host for a while, followed by maxing out 1 CPU core on the UI Server host for a while, while the loading spinner showed in the UI the whole time it was waiting for the subscription to update with the new N value.

Reproducible Example

TBC

Expected Behaviour

It doesn't seem right that dropping down the N number (leading to a relatively small number of tasks) would take so long, longer even than raising from 1 to 3.

Possibly we could also show this better in the UI.