Thanks a lot for the suggestion, running these tests has clarified the problem for me. It seems like MTA 1.5 doesn't recognize my Nvidia card and uses the Intel HD graphics instead, which is much slower obviously. Forcing MTA 1.5 to run with the Nvidia card (right click - open using powerful graphics processor) gives me the good FPS I used to have with the older MTA versions.
Benchmark on MTA 1.4: http://pastebin.mtasa.com/310975428
Benchmark on MTA 1.5: http://pastebin.mtasa.com/219679758
Benchmakr on MTA 1.5, forced Nvidia GPU: http://pastebin.mtasa.com/511273877