The clutch is controlled hydraulically. There is a master cylinder behind the pedal that pumps fluid into the slave cylinder over the transmission. When it is hard to push down, then it could be frozen hydraulics or a clutch pressure plate problem, but the fact it is also slow coming up means it is only the hydraulics. It could be master or slave. But since the slave is out in the engine compartment where is can rust, I would guess the slave is more likely. I was able to take the rubber piston seals into a clutch specialty shop and they found BMW parts that were a perfect fit, for only a few dollars. To take the slave cylinder apart for cleaning, I would suggest using the fluid pressure to pop the piston out by removing the cylinder but keeping the line attached. If there is any rust, it must be removed with fine emery or steel wool, but all grit must then be washed out. Then use brake lubricant to aid assembly and prevent future rust. If the slave does not do it, then do the master cylinder next. It very rusty and if unable to find a new one, the old one can be chrome plated back to spec.