Hi, I would love some help with this:

From when I started playing LoL on this computer until... yesterday, I've never dropped below 50-60 FPS, except when Graves Smokescreened me or when an enemy Nocturne ulted.
Starting yesterday (before the transition to the winter map), all of my games have started at 50+ FPS. Over time, the FPS fell. By the time I was 10 minutes into a game, my FPS fell to around 30 or so. By the 15 minute mark, it would be below 20.
My FPS in late game tends to have a consistent maximum of 15 or so, often dropping below 5 in teamfights.
I have not changed any video settings from the default ones. I defrag on a regular basis. I have checked for viruses (ESET Smart Security). I play LoL without background applications.

My GPU is an Nvidia GeForce GTX 485m.

Anyone have any ideas as to what may be causing this?

ever since the last update before this last one my fps has dropped overall from a constant 60 to 60-10.

ATI Radeon 5870

after the game, check process in the windows administrator, maybe there is the problem, it happends to me sometimes...
svhost use 100% cpu sometimes its annoying

I experience the exact same problem. Usually in team fights my fps is at 40-50, but after the patch my fps dramatically drops down to 10-20, even in small battles my fps drops to 30, whereas before the patch, there was non-existent fps drop.
My laptop uses an Intel HD family graphics card and a Nividia GT 520MX together (my leptop has been switched on to maximum performance).