vacuum_cost_limit
No restart — settable per sessioncost units (accumulated before sleeping)
The cost budget for *manual* VACUUM — also the basis for autovacuum's own limit.
What it does
Manual VACUUM is throttled the same way autovacuum is: it accumulates 'cost' as it reads/writes pages, sleeps when it hits vacuum_cost_limit, then resumes. autovacuum_vacuum_cost_limit defaults to this value, so this is effectively the cluster-wide vacuum throttle when the autovacuum-specific one is left at -1.
How to tune it
For an urgent interactive VACUUM you can disable the throttle entirely in that session (SET LOCAL vacuum_cost_delay = 0) so it finishes as fast as the disk allows. For autovacuum itself, prefer raising autovacuum_vacuum_cost_limit rather than this.