I recently noticed that youtube videos sometimes switched down to 720p, which made me do a speedtest on speedtest.net. This revealed, that of my 250/50 Mbit Internet I only got about 80/50, which still should be plenty for a 1080p youtube video. But the needle of speedtests "tachometer" also stuttered a lot and even froze my mouse for half a second during testing. This was caused by the CPU reaching 100% load. Speedtest should not do that, since my CPU is a Intel Core i5 2500K clocked to 4GHz (I know it is old, but not that slow).
However disabling KIS completely during testing solves this issue. The CPU keeps at ~10% and I get my full 250/50Mbits.
What is wrong with my KIS?