As painful as it is to do, In Safari > Reset Safari remove as much as you can bear of the various items that accumulate over time. I would start with the webpage preview images and website icons. The next big slowdowns could be caused by the size of your autofill files including saved names and passwords. You can edit those in Safari > Preferences > Autofill or you can flush them wholesale in Reset Safari. The final slowdown culprit I can think of is cookies which you can edit individually in Safari > Preferences > Security > Show Cookies or here again delete them all in Reset Safari.
