John Galt wrote:
I have no affiliation with EtreCheck or its developer. I do not know if it would help identify those "runaway processes" either—Activity Monitor should reveal them. Your screenshot of AM shows WindowServer taking somewhat more CPU than usual, but not so much that it should cause such an effect on performance.
Activity Monitor will definitely reveal it. EtreCheckPro will do it too. This is one area where being in the Mac App Store is problematic. I can only get information for user processes, not any processes owned by other users such as root. I really have to jump through some hoops to find those CPU usages. I was surprised that said hoops themselves even worked in the Mac App Store.
EtreCheckPro, however, can just run standard system tools like "ps" and "top" to find this information.
Considering the fact that the filesystem performance test fails at the same time, and the fact that this CPU using isn't all that high, this is probably the creation of a local snapshot. That could lock up the filesystem and cause the test to fail. It also tends to take 100% of one CPU core for kernel_task. This is probably not a kernel throttle where kernel_task is taking 500-1000% of the CPU.
It is probably normal and closely related to the low disk space on the boot drive.