uuhhhmm... If you base the game on spent time, not frames then frames won't be skipped, they just follow each other... If you base it on frames, then you might talk about skipping frames

About slowdowns in DelphiX:
In case you're developing your game for a reasonable computer today... Say... 1GHz CPU with a GeForce 2 or Radeon 7000, then the latest version of UnDelphiX should give you a good enough performance. And that configuration is the minimum I could imagine a game would be wanted to run nowadays. If your game isn't running well enough on such a machine, then you probably should think it over, you prolly have some inefficiency in there.
Please check if you have acceleration on. Have do DirectX7Mode, do3D, doFlip (makes fullscreen drawing faster).
To check if you have acceleration on, just draw a coupld of bigger pictures additively... If your framerate drops like hell, you have it off, if the speed change is hardly seeable, you have it on
About my project, I give the users the choice to use it in window of fullscreen. They can just change any time with Alt+Enter, question solved on my part
Also, about the more time consuming things like the additive or alpha drawing, I've added switches that the user can set, so the game would have a low and high quality mode. And for the very "high quality" effects, like additively drawn smoke particles, the game would look at its FPS meter and adjust the smoke acordingly. Skip it totally in low quality.

VSync makes DirectX wait with each buffer flipping untill your monitor finnishes drawing the picture one. This means that if your monitor's running at 60Hz, then your framerate will not go over 60Hz. This also will do smaller timeouts when your game's running under 60FPS, cuz the proggy may finnish drawing the picture in say... The 2,01Hz of the monitor's refresh, then VSync would probably wait till the 2nd frame of the MONITOR would finnish, and only flip before the monitor would draw its 3rd frame. Generally, it's okay without VSync, at least I don't notice tearing without it. (When the draing buffer is flipped when the monitor's only half-done with its drawing, so for example, the upper half of the monitor would contain the game's frame x and the lower half would contain the game's frame y.)

Aaaand if you're developing for a computer so low end that it does't meet my upper configuration, then you should consider learning pure DirectX on C++, bcuz Delphi wasn't made to be the most optimized game engine as it's logical... Also, maybe Direct3D's features may not even really work on such an old computer (in an accelerated way of course... The CPU loves emulating )

P.S. Sorry if I said something obvious or totally stupid