Thats probably because the lockscreen is just a fullscreen application under X11, Weyland actually understands the concept of a lockscreen and keeps things a tad more secure.
With KDE4 I often have to wait whilst the lockscreen is paged back out of swap.
Invariably I miss the fact that the disk-activity light is still lit, start typing my password and then become annoyed because it only caught the last few characters. So then I have to wait AGAIN whilst it proudly announces an authentication failure and punishes me with a further delay.
So your problem was a process going to swap (or not, Linux still has problems with disk i/o, especially going from hybernation, although bug #12309 is gone), rest are the consequences.
I get the same problem on Fedora, its irritating.