...compared to the installed one (in a Windows 32 or x64 environment)?
I suppose the installed version is faster (at least when starting up), otherwise what would be the advantage in installing? :]
Anyway, I'd like to know for sure whether there is an actual technical reason for that and also, in case there is, how much is the... price of portability in terms of performance.
Supposing, obviously, that both the installed and portable versions run from the same local hard disk.
If you're running both the installed and portable versions from the same fixed disk then speed difference should be neglible, if any, since they are basically the same app. Naturally, if you run the installed version from a fixed disk and portable version from a USB disk then the speed difference will be noticeable to varying degrees depending on the specifications of your USB hardware (but it will be noticeable regardless). The advantage of installed apps isn't in speed, it's in system integration. It's hard to fully integrate PortableApps apps or indeed any portable application into Windows without leaving traces. This is particularly important for things like default browser and default email client, slightly less important for filetype associations, such as which application to open a PDF file or a Word Document with etc (there are portable solutions for that though).