Please could you give us some details about your circumstances & repo at https://jira.atlassian.com/browse/SRCTREEWIN ? I test with quite a few repos and your performance experience with the Win beta is unusual - viewing files in the working copy should usually average well under a second. Do you have very large diffs for example? Info about the repo you are testing with would be very useful, especially if it's public. The Mac version of ST went through a lot of tuning over the years so we'll probably have a bunch to do on Windows too, your assistance would be welcome :)
Some feedback for you since you probably wouldn't find out in other ways. I used the Mac version for a while comparing against the Github client. SourceTree then insists you register to continue use which isn't unreasonable. However registration wanted me to create an atlassian account of some sort that looked like a really crappy bug tracker (ie yet another username and yet another password on yet another site that I had no intention of using). So I gave up and kept using the Github client (and the command line mostly). I had expected registration to amount to a one off name and address kind of thing.
Sure, I'll share my experience over there. The slowest part seems to be multiple file selects or working with multiple file selects (right click to add or remove from index) in the Working Copy changes and Staged Changes windows.
Steve Streeting, SourceTree dev @ Atlassian