Low upload speeds after 6-8 hours - workaround pause/resume all or app stop/start

[FloP019]

Hello! First I want to investigate the problem before I open an issue.

I have this problem since I start using the app, from version 4.6.4 up to 5.0.1 with libtorrent 1.2, same issue with 4.6.6 with libtorrent 2.0. I'm using only a private tracker.

Problem:
After a few hours (6-8), upload speeds slowly decrease from around 40MB/s until it reaches 0kb/s - 1-2MB/s interval and remains there. With that I barely get 5GB upload per day. The workaround is to pause all torrents and resume all getting around 20MB/s upload speed or 40-60MB/s interval if I stop/start the app, with this I can get up to 2TB/day upload. Nothing relevant in log. Enough peers for torrents.
My system is fine, and the app works great, 60-110MB/s download speed and 40-60MB/s upload speed, stable speeds, port forward set ok, gigabit connection, no issues with RAM or swap, download/upload from SSD, enough free space, no changes to settings in between.
I have around 30 torrents, ~300GB.

Same results with default settings as with custom settings, clean install or update.

System:
QNAP NAS 431X2, 8GB RAM, Quadcore processor
OS version:
QTS 5.2.1.2930 build 20241025 (latest)
App details:
qBittorrent v5.0.1 WebUI (32-bit)
Qt: 6.8.0
Libtorrent: 1.2.19.0
Boost: 1.86.0
OpenSSL: 3.4.0
zlib: 1.3.1.zlib-ng

On image 1, upload speeds around 200KB/s (for hours), after pause/resume all, stable upload speed around 20MB/s.

On image 2, upload speeds around 200KB/s (for hours), after pause/resume all, stable upload speed around 20MB/s for 1h, after app stop/start around 40MB/s.

On image 3 after pause/resume all, stable upload speed around 20MB/s for 1h, after app stop/start around 40MB/s.

Please advise how to investigate this issue, I'm a Linux beginner and i cannot make builds (using builds from MyQnap repo)