Backup causing network latency spikes on VMs

[xPakrikx]

Hi

I am experiencing network latency spikes on VMs when backup job is running on same Proxmox VE node (PVE 7.0 + PBS 2.0) sometimes up to 800ms.
I tried isolate vHDD with second storage , disable RAM ballooning and test network with iperf3, but none of this options are causing problem.

I use Dell R620 with 32 x Intel(R) Xeon(R) CPU E5-2670 0 @ 2.60GHz (2 Sockets) and CPU usage is about 10%, IOwait max. is under 2.5%. Where could be the problem? cpu scheduler ? During backup CPU load is about 15%

Any idea ?