When the landing-gear and cargo-scoop "50% speed" change was first added (beta phase), it was able to slow you faster than any other method.
FD devs went on record saying this was absolutely NOT working correctly. It was supposed to do nothing more than you could do yourself with a setting of 50% throttle.
This was supposedly fixed in a subsequent revision. For the landing-gear, the change was in-your-face obvious. Literally. You'd have a control tower smashed into your face if you didn't moderate your insane docking approach speed (learned during the hilarious "landing gear air brake" phase, lol).
If either LG or CS deployment slows you "outside the normal flight envelope", it's a bug. If you have empirical evidence, like a video, upload it to YT and raise a ticket.