Hum, i'm almost sure it's the hard drive then.
When the hd is full or nearly, fragmentation is becoming really important, since the os is putting all the files/bytes in the small holes left, scaterring everything.
That's even more problematic with rs on svn because the number of files is simply huge. (About 150k, which is about 10 times the average number for a game, the matter is make worse with svn which basically doubles the number of files.)
Conventional hard drives (ssd are a bit different) are consisting of a disc spinning, with a reading/writing head.
When a hd is defragmented, all the rs2 files are kept together, and the head does not have to move a lot or often to read the data, this is called sequential read.
On the other hand, as it's probably your case, the data is scaterred all over the disk, and thus the head costantantly have to move everywhere to fetch the data, that's random read.
Since we have to obey the law of physics, random speed is necesserely slower than sequential.
The game is accessing the hd mainly during loading time (start and battles), but it does not during battles, so there is no performance loss in that case.
Tl;dr Try to make some space, and defragment the disc.
Concerning the difficulty, would it come from having less money than before?