My PRS-300 has taken as long as 30 minutes to recover from being plugged in to my computer. During that time, the silly circle-arrows icon displays, but often do not move for minutes. This happens even if I do nothing to the reader while it is plugged in.
Experimentation showed that the recovery time depends on the number of books on the reader. 10 books, very fast (seconds). 500 books, very slow (many minutes!). As a result, I now limit the number of books on my reader to those that I might be interested in reading within a few weeks.
Also note that I never charge the reader from the computer using USB. Inserting and removing the PSP round plug does not cause the reader to waste my time while it contemplates its navel.
I can still use my computer as the power source, because I have one of the dual-ended cables (USB and PSP 4mm).