I guess it depends on what's actually slow.
If the computer itself is generally fairly slow then an SSD might help.
If it's slow loading the app, slow loading plugins and stuff like that then an SSD might also help.
If it's slow loading run of the mill web pages then this is more likely a function of the Internet connection and whatever's going on over that connection (background downloads, torrents, updates, etc). In this case, adding an SSD may make very little difference.
David