Besides the obvious thing of showing a process blocking on a read() from a source of entropy, you could also change the minor number on /dev/random to temporari

Besides the obvious thing of showing a process blocking on a read() from a source of entropy, you could also change the minor number on /dev/random to temporarily make it the same as /dev/urandom. No code needs to be re-compiled or added to the system. Just make everything restart (so it grabs the right /dev entry with the new minor number) and see if it changes. That should tell you if it's actually a lack of entropy, or another process undoing the CPU governor, or something else entirely.