It's not really that great, in sv ou at least. This tier is filled with so many offensive threats like Chinese Weavile and Nemo that sd Garchomp gets eclipsed not to mention that its own hazard machine set is really great in the current meta. It's not its speed that's the problem, it is the fact that there are simply so many Pokemon that poses a threat offensively that are much better to use. Another problem for offensive Garchomp is its dragon stab. Outrage and dragon claw are simply not great picks anymore. You do not want that Gallade and Gardevoir fusion to come in for free and dragon claw is just too weak. Last gen, it had scale shot that even though it was heavily rng dependent, at least it allowed Garchomp to be a terrifying sweeper. Overall, it's probably a B+ ranked set, A- at best. The opportunity cost of not being a hazard machine is simply too high given how it can brutally punish so many contact moves in the tier
What makes sd Chomp interesting however is terastal. That ice and fairy weakness suddenly become resists with either tera fire or steel and fire allows it to melt Corviknight. However, as I mentioned earlier, the lack of dragon stab is the real problem and stone edge isn't a good move right now with the Past Donphan being so good. This set also gets walled by a certain unaware catfish, Dondozo which is really bad. Ironically enough, even mix Garchomp is probably better than sd because it can draco the likes of Past Donphan and Dondozo while melting Corviknight as well. In sv ou, I haven't seen much of sd Garchomp as its usually used as a hazard machine
It's probably a different story in doubles since those don't require much of hazards but it would probably depend on the team
As for your last question, no, 102 speed is not too slow. SImply put, there are many other factors that play into sd Garchomp not being that great. Not to mention terastal can create guessing games with so many viable options like ghost, steel, fire or water
