My site when I migrated to 1.7.8.8 was running SUPER SLOW, I regretted updating.
But then I rebuilt all image/thumbnails in the back-end, and it became lightning fast, and a huge relief for me. Try this. It might take a while if you have lots of products. I regenerated all images/thumbnails of all my products with the tool, I kept getting a 503 timeout, but I did all of them.
Go into your back-end, clear/re-build image index, search indexes, clear cache, ETC ETC and CTRL + F5, see if this works. If not, disable your theme and use the default one. It should load fast - if not, you've got a bad module. Disable all your modules, CTRL + F5, try again.