Been testing this with friends to verify claims that this is only happening on Summoner's Rift. Currently, we've been doing some Dominion to test if it occurs upon that map, but it doesn't seem to be a case that Crystal Scar causes the same FPS issues. Since this came immediately with the Graves patch, I think it is likely that the issue is located within the Summoner's Rift - Harrowing map.
Can anybody confirm this, potentially by testing on Twisted Treeline or other Summoner's Rift maps to see if the same FPS loss happens?
Note that this is specific to the issue of FPS loss following the Graves patch, not the other FPS issues starting 3-4 patches back.