I've never understood that "explanation". The fairly obvious best method would be to only compile the app the first time it is run, or at installation time only, and keep the compiled copy around thereafter.

But the stock explanations for the two runtimes present each as "all or nothing" --> Dalvik being "nothing" pre-compiled, and ART being pre-compile "all".

Right now, with ART, if I upgrade the phone or simply wipe the cache, there's a 5-20 minute delay on the reboot while the system tediously recompiles every last app.

So I use Dalvik instead.