So the problem is on the SSD. The KIOXIA TC10 doesn't have DRAM and so the speed can fluctuate drastically. I replaced another Samsung 860 EVO SATA SSD which has 512 MB DRAM. Now the speed can goes up to 550/520 Mbps. Turing on FileVault decreases the speed also. After turning on FileVault, the speed drops below 500.