Do lubricants slow down sperm?

THEY CAN: Infrequently is there enough lubricant to cause this and many are not very thick at body temperature. The semen liquifies in the vagina after a few minutes allowing the sperm to swim more easily.

Depends: Some do, and others don't. See what it says on the package or contact the manufacturer. If you're using birth control pills, it probably doesn't make any difference.