I also tested OS 4. Beta 1 works well in my 3G. I JB my iPhone and enabled wallpapers/multitasking. Both options doesn't slows down my device. Sometimes when you swipe it had some moment of sluggish but hey it is a beta so here and there a little tweak and it works flowless.
I open some default applications and even then it still working fine. (MS)
This is my wishlist:
* Better notifications for SMS/Mail/missed calls and apps etc.
* Shortcuts to enable WiFI/BT (Widgets)
* A quicker way to see the calendar (ex. lockinfo)
* Tasks (ToDO)
You may have enabled multitasking on your 3G, but without any multitasking apps, you won't be running any of the services. Therefore it isn't a fair test on the speed.
Wallpapers are hardly going to slow things down. Once the wallpaper is rendered it is there.