Although it’s hard to believe and took me a week to figure it out, I tracked down the problem to intermittent very slow NVMe read performance on the Win10 machine. The problematic drive is 4.5 years old Gigabyte Aorus 4 2TB, GP-AG42TB with a Phison PS5016-E16 Controller. According to Google searches, it is a well known issue of the Phison controller SSDs - reading old data (sitting on the drive for months or years) results in extremely degraded read performance (around 8-10MiB/s).
Anyway, not a TrueNAS or SMB issue at all, I’m sorry for misleading anyone.