FPS drops tremendously as game time extends

I had a custom game for hours just for the fun of it and to test how powerful Gathering Storm is in the very late game, so the game lasted for like 3 hours... Starting from 40 min my fps went from 100+ to 70-80, then it further worsened after around 1 hour to 50-60, until after 2 hours the fps was 30... and by the time I ended the game the fps was like barely playable 15 - 20... Running with a single card (980ti) and saw the gpu usage was within the same bands the entire time... was playing with 9 intermediate bots... The bot veigar got 5k+ ap and started spamming W rapidly, wonder if that's the cause of the slow down But before I could do very long games without FPS issue Could you please look into this? Thanks!
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