Remember all those line format
specification variables? gnus-summary-line-format,
gnus-group-line-format, and so on. Now, Gnus will of
course heed whatever these variables are, but, unfortunately,
changing them will mean a quite significant slow-down. (The
default values of these variables have byte-compiled functions
associated with them, while the user-generated versions do not,
of course.)
To help with this, you can run M-x gnus-compile after you've fiddled around with the variables and feel that you're (kind of) satisfied. This will result in the new specs being byte-compiled, and you'll get top speed again. Gnus will save these compiled specs in the .newsrc.eld file. (User-defined functions aren't compiled by this function, though—you should compile them yourself by sticking them into the ~/.gnus.el file and byte-compiling that file.)