I'm the developer of Tumblr for iPhone and sorry to hear this. I know that web views have their drawbacks and we're working hard to make the app faster and more stable in every way.
- cant long press to copy a tumblr link (heaven forbid I'd like to share a link OUTSIDE of the app - cant post YouTube urls in video post - reblogs won't let you delete prior comments by others (sometimes you have to clean up cluttered reblogs) - you took away options to open in safari in many instances as well.
I strongly encourage any users with feedback or feature requests to hit up my ask box: http://blog.bryanirace.com/ask. We really are listening and plan on providing the best mobile Tumblr experience we possibly can.
Edit: Worth noting that we added post sharing options in 3.1 (long press on the 'Like' button).
Please try going native, the performance increase would be in the order of magnitudes.
Are you the only one working on the app?
Tumblr content can be quite diverse and often contains arbitrary user-inputted HTML, numerous inline images, etc. It's non-trivial to render but as I mentioned, we're working hard to provide an even better mobile experience.