I could be wrong but with 8/138gb free, your disk could be getting heavily fragmented. Basically there's not enough space to save files in one piece so the big ones are being scattered around the whole drive meaning it takes ages to read them. Try running disk cleanup first, remove temporary files etc. Try to get rid of as many programs you don't use as possible. After you do that, it'd be a good idea to defragment the hard drive. I use AusLogics Disk Defrag, it's free and it's quick.
+what the others said.