Looks like I had too much time ... so I added slow motion and bullet time modes to D2X-XL. Slow motion makes everything slower, including the player. Speed works in conjunction with slow motion and speeds the player up to normal speed. You will need to have the energy->shield converter to use either mode. For speed, the afterburner is required, too. Above that you need to have more than 10 energy for slow motion and more than 20 for speed, since the latter consumes twice as much energy.

There are two new key bindings in the keyboard configuration menu for the two modes. The first one toggles slow motion, the other one toggles speed. Hitting the slow motion key while in speed mode will turn speed off. Hitting it another time will turn slow motion off. Hitting the speed key while in slow motion will turn speed on. Hitting it again will turn slow motion and speed off, returning to regular game time.

Did that sound a tad complicated? Let me explain it this way: If one of the modes is active, hitting the other mode's key will activate that mode, and hitting that key again will return to regular game time. Better?

Slow motion and speed are shown in the cockpit to the right of the score display. 'M' is for slow motion, followed by the current slow down, and 'S' stands for the speed, followed by the current speed up. Additionally, a HUD message telling whether you are speeding up or slowing down is displayed, and an appropriate sound is played (sound only if SDL_mixer available and enabled).

For technical reasons slow motion and speed are only available in single player.