First of all, please check that it really is LeechBlock that’s slowing down your browser! Disable the extension and see whether the problem disappears. If your browser is still sluggish, LeechBlock isn’t to blame.
Second, make sure you have the latest version of LeechBlock installed, because version 0.9.11 introduced some major performance improvements.
If you have a large number of tabs open in your browser, LeechBlock can sometimes slow things down. (It depends on the specifics of your system.) Under normal operation, LeechBlock reviews all open tabs every second, to keep track of how much time is spent on the sites listed in your block sets and to determine whether any sites should now be blocked. If you have many tabs open, either (1) reduce the number of open tabs or (2) select the option Process only active tabs (under General tab in Options).
If you think LeechBlock is slowing down your browser even though you don’t have a large number of tabs open, please contact support.