SoftRaid Multiple Hangs During Certify + Slow

I just completed certifying five 6TB HDDs, but after hours to days SoftRaid would freeze up. I was able to resume the certify process each time, so after multiple (I estimate about 5-6 sessions), was able to finally finish. I'm running 5.7.2 on Mac OS 10.14.1.

Now I have another problem: the 6TB x 5 RAID5 volume (optimized for digital video) seems much slower than my previous 3TB x 5 RAID5 volume. I just tested using BlackMagic Speed Test and the new volume does 333 MB/s write, 411 MB/s read. IIRC, the old 3TB x 5 ran about ~490 MB/s write, ~900 MB/s read. The 3TB drives were 5 year old WD Blacks, the 6TB are Seagate ST6000DX000 7200 RPM drives (brand new but 4 yr old).