My Eclipse was getting slower and slower over time. Tips I found on the Internet did not help. What I did is completely deleted my workspace, created new one and reimported all
Not sure about newer versions of eclipse, but in 3.1 settings in
Preferences->General->Workspace->Local history
did not work for me. I had it set by default to 7 days but files were kept for 4 years. And I guess other people here had the same issue.
For me it helped to remove history files manually from
.metadata/.plugins/org.eclipse.core.resources/.history.