Even when Web Content’s CPU usage is at a manageable level, its Real and Virtual memory listings can be quite high. When this occurs, everything in Safari (and sometimes everything on your Mac) typically slows to a crawl.
Some users suggest that Safari add-ons are the culprit, although others speculate it is an inherent memory leak in the Web Content process itself. If it’s the latter, adding more memory to your Mac will be of little, if any, benefit. Rather, it is something Apple will have to fix
in a Safari update. If you quit and relaunch Safari (without reopening all its previously open tabs and windows), the symptoms should temporarily go away.