I actually think it will slow down the phone because the sdcard is slower than internal memory. Therefor any app on the sdcard is going to be slower (maybe even problematic) than if it was on the internal memory. I kinda think of it like a PC, install an app on the internal SATA hard drive. Then install that app on an external USB drive. USB is slower than SATA, and thus will have degraded performance in comparison. With PCs oftentimes this won't matter to much, since USB is often faster than it needs to be for apps. But some of the more intense apps, specifically games, USB may be a limiting factor at times.
Apps2SD is really only beneficial to older phones with low amounts of internal memory (at least from my perspective), like the OG Droid. Droid X, D2, and most other newer phones have more internal memory making moving apps2sd rather pointless. The OG Droid only has ~100megs for apps, and there are some rather large ones out there. Google Earth and Homerunbattle3d both take up ~20megs of space each. There are some other fat apps as well (the FiOS remote and DVR app are bloated too
). The problem is, as I said resource hungry apps (often the big ones), run poorly on the sdcard. So you would have to move 10 little apps off, and hope they don't suffer performance in order to make room for one of those big apps.
Since apps2SD allows you to have more apps installed by allowing you to put some apps on the SD card. More apps in theory is going to slow down your phone. Just because you probably are going to be running more apps, maybe. As long as you are under the recommended memory allocation and apps aren't starting themselves for no reason, you shouldn't notice a difference between 20 apps and 200 apps. But in theory this could be a bad thing.
This is at least my perspective and experience with apps2sd. I haven't really tried the Froyo version, because there was only 2 apps that I can move to my sdcard that I have in my quick scan through my manage apps list. SetCPU, which I probably would never notice it was on there since it doesn't run a whole lot (I don't run the widget). And another was one of the dinky games I have, Sudoku or a card game, something really simplistic. Which once again I probably wouldn't notice if it was slow or not. My experience with Apps2SD came from Cyanogen back in the 2.1 days. It is implemented a little differently than the Froyo version, but still I would think results would be similar.
I just keep myself under the space constraints and have learned to live without Apps2SD. Also with root access you can push a few files into the system partition, without any performance hit.
