The common wisdom is that EXT2/3 are not affected by fragmentation, but
without much real-world proof that this is so. The EXT2/3 filesystem
metadata was designed to be not much affected by fragmentation, but that
says little about the file data. I read an article / webpage (that I can't
find right now) by someone who decided to experiment with new and used EXT2
filesystems, and found a substatial slowdown. He was inspired to try this
because he noticed that his computer sped up when given a fresh filesystem.
You could try backing up and restoring to a fresh filesystem. If you
spring for a new computer you'll back up and restore to the new computer.
Either way you'll get a fresh new filesystem.