It really depends on the situation, and what you're facing.
A space station usually have 35-40% more usable space for defenses or weapon compared to most ships. Between engined, fuel, additional crew space etc. Even more if the warship has to be jump capable, and so has a jump engine. And you also save a lot of gallicite, which is usually a rather scarce mineral all things considered.
As said, the lack of mobility can be a problem in terms of dictating the engagements, but I would not say the speed is a problem. Unless a warship is VERY fast for its tech level, which is a big investment on its own, speed will not generally cause a very large portion of the enemy weapons to miss. And a space station, due to the extra usable space, can mount a lot more PD/armor/shields.
Also, a mobile force can move, as the name says, while stations are usually just for protecting planets or jump points. So you lose flexibility.
As said it depends on the situation. Starbases can be exceptionally efficient in the following situations imo
-PD/AMM stations protecting an important planet. Amazing amount of PD/AMM "power" for the tonnage
-Beam station protecting an important planet from beam warships/fighters. Will likely need particle lances to be truly effective, or good tech lasers, but once again, amazing stopping power for the tonnage
-Missile stations protecting a planet. This is the closest equivalent of the VB6 aurora PDC, and it's a lot of missiles avilable for the tonnage
If used for jump point defense instead, you have two possibilities imo
- the aforementioned missile station, which can be very powerful
- a "carrier" station, to park somewhat further away from the jump point. Load it with a TON of fighters, enjoy the show
PD or beam stations don't make much sense for jump point defense, because the enemy can quickly flee outside of your range, so they lose a lot of their meaning.