Beta 3 is always installed in flash (there is no option to install it to the hard drive). With pdaXii13, you can install it to the hard drive depending on what version you get (The akita version, aka Sally, is what you want since you want to install to flash).
If you're content with beta 3 then you don't have to do anything (except maybe apply some fixes to get everything working), or you can flash to pdaXii13 Sally and have those fixes done for you (if it's still slow, try disabling SCIM if you don't use it -- and there are lots of other tweaks in some of the pdaXii13 threads in the pdaXrom forum).