Is it ever faster to slide instead of sprint when travelling over flat terrain?

Yes, you can move faster by mixing in slides than by just sprinting.

I tried both in the training room and recorded the results:

Sliding is indeed faster, even on a level surface.

I have tested this several times now alongside teammates who are only sprinting.

Also, if you jump at the end of your slide you can carry that slightly faster speed for a bit in the air as well. I've even found that up a slight incline you can still get a little speed boost if you slide and jump right away.