If you use Lightspeed for any DPS spell outside of Gravity spam it's actually a 25% loss since you lose 25% DPS due to the effect of Lightspeed but can only cast the same number of spells within the same time frame.

Gravity is the exception because it's normally 3 seconds to cast but only a 2.5s recast, effectively allowing you to get 5 spells within the same period of 4. Therefore the loss in DPS isn't as notable when using Lightspeed specifically for Gravity but will be quite noticeable on any other DPS spell.

If you happen to have Lightspeed up that requires you to do a movement mechanic, yeah, I'd probably just keep it up and have 75% potent DPS is better than 0% DPS but I wouldn't go out of my way to pop Lightspeed just for DPSing on the fly either if I can help it.