Suddenly the solution appeared to be:
hdparm -W 1 /dev/sdb
This turns on internal write caching in SSD. RAID1 resync speed increased greatly with this option (from 20 MB/s to 350 MB/s), "iostat 1" also shows the increased throughput, so it's not a fake.
I've also been experimenting with disabling/enabling write caching (-W0/-W1) at a machine in Hetzner with Intel SSDs. Turning off write caching (-W0) reproduces the same effect of overall slowdown and slow RAID1 resync. Turning it back on restores everything back.
