Why tumblr is slow? Tumblr is slow because they're getting too much traffic to handle, and they need to upgrade. Facebook is hardly ever slow and either is WordPress or Blogger, so what's Tumblr's problem?
What makes Tumblr slower than Facebook, WordPress and Blogger
Facebook and Blogger(AdSense integration) sell advertising and can afford more server cpu. WordPress.com allows you to self-host so the server burden is handled by whichever hosting company you select. Tumblr is a great service unfortunately they refuse to sell advertising, and they don't offer self-hosting. You would think that Tumblr would offer their product as open source if money isn't the main objective, but they don't. Tumblr's setup with this heavy workload and limited finances, is a plan for failure. Actually Tumblr is starting to offer a form of advertising called 'Tumblr Radar' but they're going to have to step it up a bit if they want a return on investment.
Developing Tumblr themes is even slower. I tried to develop a theme for Tumblr, and the wait time made it unprofitable. No wonder there're few Tumblr themes out there to use.
Avoid busy server times
The best thing you can do to avoid a slow Tumblr experience is to avoid busy hours. Most people use Tumblr Tuesday through Thursday. The weekends are the least busy times, Saturday being the least busy day. Make your updates before 8am and after 10pm..