How to investigate 99% CPU usage by w3wp.exe

In recent versions of Process Explorer, the CPU usage percentage is calculated differently. According to Mark Russinovich, the author of Process Explorer:

Process Explorer shows actual CPU usage now based on cycle counts. If you have speedstep or other CPU frequency throttling enabled that will cause work consuming some number of cycles to be calculated as a higher percentage of available cycles per second.

If you prefer the old CPU percentage, use a previous version of Process Explorer (v14.01). You could also ask for the old CPU usage percentage in the feature request thread.

Note the "show one graph per CPU" option shows each CPU's usage without accounting for speedstep. However, the other CPU percentage displays are not affected. You can also disable your processor's speedstep, but that seems a little extreme...

Not really a definitive answer but here is a workaround which basically disables the Web Proxy Auto-Discovery Protocol (WPAD):

Put 255.255.255.255 wpad. in the host file C:\windows\system32\drivers\etc\hosts and reboot. The process should now stop hogging the CPU.

See https://stackoverflow.com/a/41048991/3515906