Please upgrade your Waitress from 1.4.2 to 1.4.3
Pylons Project released a new version of Waitress to fix a bug in the regular expression that was used to parse the HTTP headers.
The bug would allow for catastrophic backtracking which would cause the waitress process to spend 100% CPU time in attempting to match the regular expression.
As Plone 5.2.1 uses Waitress 1.4.2, it is recommended to pin it to version 1.4.3 on your buildout file
[versions] waitress = 1.4.3