Gargoyle Forum

I agree that it is CPU cycles probably. There is similar reports on some devices on openwrt forums.

What could cause those peaks if the PC just idle and this is the only connected device while I test?

CPU peaks during pings?
I do not see such increasing in ping time even when my bandwidth is almost saturated (well, with QoS enabled).

It doesn't have to be saturated. The CPU can be busy doing another task.
Run the ping test on one console and monitor "top" on the other. During the high ping spikes, does the CPU idle time also drop? If so, you've got something holding up the CPU.