In a Windows 10 Pro 32 system, I had25-40% CPU for Antimalware Service Executable and another 25% for Malwarebytes.
In all the CPU was running 100% with other things.
I normally run Widnows Defender and Malwarebytes Premium together on lots of computers. So this is NOT a problem.
And, turning off Windows Defender or Malwarebytes is NOT going to be the solution.
So many posts here go in that direction it seems.

I'm sorry that I can't be more definitive but here is how this was solved:
I had about 6 command line .bat files running continuous pings to various addresses that would log traceroutes should some number of pings be missed in succession. These were originally written for Windows XP and 7 and there were timing differences between the two. Mostly they just run the pings and that caused the timing issues.

Closing the looping .bat files got rid of the Antimalware Service Executable and Malwarebytes heavy loads. Now they are running under 1% each.

It appears there is enough interaction to cause the high loading. So that's where I'd be looking.