Migrations to one node very slow

I'm sometimes seeing migrations slow down for some reason. This time I turned on maintenance mode on one node—the first few VMs migrated within minutes (avg. speed around 130 MiB/s), but there are other migrations that hover around 50-200 KiB/s.

Any ideas what could be the culprit here? The migration network isn't used for anything else.

Edit #1: I've changed the 'maximal workers/bulk-action' setting from 4 to 5 and the fifth migration also finished within minutes (193 MiB/s)—while the...

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