From a terminal, type: top
Top is a command line process monitoring app that can give you an idea of what is running on the system and what type of processor cycles, etc. that the running processes are using. It may give you an idea what the culprit is.
9 times out of 10, some process is running away with the machine. It may be that there is something buggy in your installation (or bugs introduced in a package that now has memory leaks, etc.), or that some process is running that you had no idea would be such a resource hog (or your setting have caused it to be such).
PS: you exit top with the Q key.