To check the maximum allowed RAID rebuild speed:
sysctl dev.raid.speed_limit_max
The default limit is low for NVMe; it can be safely adjusted to a higher value, eg.:
sysctl dev.raid.speed_limit_max=600000
A crazy high value can be set for the initial sync (if server is unloaded):
sysctl dev.raid.speed_limit_max=6000000000