Potentially yes, but unlike in road biking, in mountain biking the main thing slowing you down isn’t wind resistance. The main thing slowing you down is you using your brakes. You use your brakes when you feel you are going too fast to ride through something without crashing because you are out of control or scared. If you can do something to give yourself more control, and feel more confident, you will ride faster. This is the same reason someone can ride faster on a bike with long low and slack geometry, or even ride faster with slower rolling but higher traction tires. Neither of these outright make you go faster necessarily, it’s that you don’t slow yourself down as much. The RR stem increases your control and confidence so that you can ride faster. When you do need to be more aero though, you can still aero tuck much the same as you can on a traditional stem.