Another weird thing. If I run it from Delphi I have no slowdown.
If I run it from the Explorer I have the slowdown.

hmmm It seems it's not my code.

I tried a sample that comes with DelphiX (the shooter) and modified the title screen to display the framerate. If I run it from Delphi it's stable and if I run it from the explorer there is also a slowdown.

P.S.
No other application is running
I have XP