Where are you seeing lag? I don't see any lag navigating around and running mail, web browser, etc. The Xbox Music app seemed to slow things down a bit, but that is fixed in the 8.1 preview. Maybe if there is a specific use that is slow for you, there may be ways to speed it up. I also found that accessing files on a SD card was noticeably slower.
I installed Windows 8.1 using the workaround method (had to edit reg because it wouldn't work otherwise - I'm in Australia) and it just seems opening things are laggy. Another issue I have is the wireless drops out frequently and I can't seem to send messages from my outlook exchange webmail
Glad you found something that works for you. 8.1, as a beta, has some issues. I fixed the mail thing, but uninstalling and reinstalling the mail app. Some apps open much fast with 8.1, some seem to be a bit slow. If seen the same behavior on pretty much all ARM based tablets. The iPad2 I used to have was the same - slow to launch compared to a full-powered desktop, but once open, everything was smooth. Most of my apps open in just a few seconds though - it hasn't been an issue for me, but it seems to depend on the app.
I'm wondering if it was the 8.1 beta that was the issue. I had to hack the reg to get it working on my aus machine as well. I really needed outlook for work though. But am glad to have made the decision (would have cost me a little less $$$ if I had decided that in the first place!) but oh well