Gradually dropping in FPS...

Hi there! I have an unsolved mystery which drives me crazy. I recently bought an used PC to play league of legends on. I also play Overwatch but was I wasn't sure it could run it. BUT! It does run Overwatch! Even on Ultra settings it keeps a stable framerate (60fps) Now here is the issue: League however... it starts off with 140 fps on high, except shadows which is on low. But as time flies by the fps slowly keeps dropping. From 140 to 120 to 90 to 60 and eventually 25 in teamfights. Now I know my pc isn't the best with a Xeon X3330 and GTX 750 and 4GB ram but according to Riot's system requirements it should be good enough! I thought it might be the ram but league requires 2x LESS RAM than overwatch does yet overwatch runs perfectly fine... But I must note that in Overwatch I do get screen freezes sometimes(rarely), but no drops in fps. I reinstalled lol a couple of times but no succes. No heating issues, cpu and gpu runs at 35-40 C during Overwatch/LoL (Perhaps a bit too low?) I think there could be 2 problems here: 1. the RAM is too old. DDR2 @ 800 or 600 mhz~ 2. I have installed windows 10 OS on my 40GB SSD, but league is on my Harddrive which is very, very old. I used run windows on that harddrive but the harddrive usage was always @ 100% I'm almost sure it's the 2nd one. BUT WHY DOES IT RUN OVERWATCH SO BUTTERSMOOTH? I play league since 2012 and I know for a fact that every toaster/laptop i had ran league easily on 100 fps. Even the oldest laptops with integrated graphics. What could be the issue here? Any tips and help is appreciated!
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