Torrenting directly to 4TB HDD basically kills it

Hello,

first of all i'm not sure if that's strictly hackintosh problem, but i have to start somewhere.

PC:

HP 800 G1 Mini

i5-4590T

8GB of RAM

256GB NVMe APFS

4TB 2.5" ST4000LM024 5400RPM HFS+

OpenCore 0.6.3 with latest Big Sur 11.1

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Problem:

Transfering anything to that HDD from internal drive or external is giving me full speed (100MB/s+).Once i start torrenting directly to that drive, i can't even get 10-15MB/s most of the times, Disk Usage is showing nothing wrong (definitely not 100MB/s), and everything related to that drive is freezing. Transmission, Plex, Roon, simply even exploring is hard.

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Now how do i assume it's not hardware issue?

Using this same exact HDD, on the same PC but on TrueNAS booted instead of MacOS is working perfectly fine. HDD formated in ZFS, running Deluge with 10+ big torrents (4k movies) = full download speed ~30MB/s, and no problems with Plex for example.

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It's the same with Transmission, Deluge, qBitTorrent, with allocation on and off etc.

Had no problem with 512GB SSD that i've used before (but was too small).

Where do i start troubleshooting?