Why is my computer running so slow in Safari? Also tried Firefox, no improvement.

Who is your ISP (Internet Service Provider)? I would suggest you contact them to run a speedtest over the phone. Otherwise, open up safari, click on the safari word next to the apple logo, click empty cache, then click safari word again, then click reset safari and tick all the boxes and reset.

Maybe your internet provider is having issues? If you feel it's actually a computer issue, try it in a test user account. Apple menu >> system preferences >> users and groups (or accounts) >> click the pad lock and unlock it >> clikc the + sign and add a new user. Apple menu >> log out >> log into new user. See if that helps. If not, likely your internet provider. If it runs fast again, then you should try emptying safari's cache, and reset it. You could also clear your plugins folder at ~\library\internet plugins and \library\internet plugins.