Recently I noticed 100% CPU usage on my Guacamole installation. Its a Debian 8.3 running Tomcat 7 version 7.0.56-3+deb8u1. It took me about 1 hour until I found the solution. It seems to be a bug related to threading (maybe Guacamole is keeping threads?) that dates back to 2009! See https://bz.apache.org/bugzilla/show_bug.cgi?id=47088.
The solution is easy and solved my problem right away. In server.xml find your connector and add
connectionTimeout="2000"
as seen in the bug report.