Network transfer speed slow when MinimServer is running

[ElfjeTwaalfje]

It was an update to 2.1 which means manual install. I did the update hoping it would solve the issue, meaning I had the same issue with the previous version. I waited until all settled down. Anyhow I did the test again. I ran below scenarios twice. What surprised me was that CPU was much higher at full speed (MinimServer off).

Edit: I also toggled Synology Media Server on/off. No effect on speed, always fast.

Stable means:
CPU 30% (likely due to monitor)
Mem 36 stopped, 51 running
Network few bytes/s
Disk 0%
Volume 100%
Shortly after MS startup clearly increased CPU and Disk.

With MS running, waited till all stabilized, then during transfer:
CPU 30-50
Mem 50
Network 25-30Mb/s
Disk 20-40
Volume 100

With MS stopped, waited till all stabilized, then during transfer:
CPU 90
Disk 30-40
Mem 36
Volume 100
Network 70, spikes up to 85