It does not seem slow at all for me. In the options menu it rates my computer between 1 and 2. I use 32 bit color in fullscreen, with an E6600 cpu (Core2Duo 2.4GHz) and Nvidia 8800GTS videocard. I'm going to try it on my laptop soon too, that uses an integrated Intel X3100 videocard.
As for the other issues (no help, game hangs on purchase), I've noticed that when clicking the help button it only prints out that I should refer to the help.html file. No big deal, though a popup would be nice (or even actually opening the help file
When clicking on the purchase button the first time the game crashed, and I had to kill it. When I tried a second time just to confirm, it actually opened firefox and went to the purchase page. I haven't been able to make it crash since then.
I've been playing for a while now, and so far it seems pretty stable. No crashes so far (exept the purchase thingy), and I haven't noticed any graphical or game bugs.
Edit: Now I've tested the game on my laptop too, with a T7100 cpu (Core2Duo 1.8Ghz) and X3100 integrated graphics. It was playable, but noticeably slower than on my other computer. The graphic card is not a very good one though. But the game rated my laptop between 0 and 1 oddly enough
The graphics also looked quite bad, since they became blurry and stretched when running fullscreen on a 1440x900 screen.