I'm sorry but I don't really know where to look for you problems.
I use my N900 at 600mhz and the only situation I can see the lockscreen when unlocking using the hardware button is:
-Open the stock media player and play something (probably other landscape only program will do the same)
-Lock the n900
-Press the power button and let the lockscreen rotate into portrait
-Unlock with the hardware key
These way I can see the lockscreen (sometimes, not even always) for less than a second, probably due to the rotation (the stock media player is only landscape and the lockscreen was rotated to portrait).

Do you have something going on when locking/unlocking like lockdaemon?
Did you try to change the theme? I'm using the default one.

Btw I installed the speed patch and I can confirm that this solves the "2 seconds lock" when the power button is pressed.

@romiiio: well, in conky (or with the top command) you can check what process is taking that 7/8% of cpu.
The problem with the slider is a known problem (read the first post), but as I said the speed patch seems to solve it.