Most common is heat related. You can find tomes on the web about this. Next up could be memory leaks. Your post doesn't tell much so you're left to figure out which it is.
I haven't seen this question before. After my computer has been on for a few hours the results of the test are bad. If restart the computer and run the test my speed increases drastically. The test goes from about 15 to 60.
Has anyone seen this before and if so how do you make the speed more consistent?