Wait, are you saying that because technological progress is too fast, technological progress is slow?

Yes. It's because technological progress is fast in general, halting a particular avenue of development down can cause it to never be pursued again.

Technological progress isn't a line, it's a graph. Someone patents an invention and decides to sit on it, and by the time the patent expires, the entire technological environment has shifted around in a way which makes that invention worthless, and follow-up work unlikely to happen. Whereas, were it not for that patent, the invention would had people extending it further, perhaps generating even more immediately useful technologies.

In this way, a patent - in particular, a defensive patent, taken to prevent anyone from developing a technology that endangers your business - is closing off avenues of progress.