Your question makes perfect sense. It's your therapist who has the wrong approach. Sadly, this happens with joint replacement patients more than it should. Going slower will NOT reduce your ability to recover overall. In fact, if you're pushing to the point of pain and swelling, going slower may just help your recovery progress in a more linear pattern (with fewer setbacks). From our experience with these gung-ho therapists, you won't get very far trying to talk to them about this. You'll probably only have two choices.....insist that they ease back on your exercises or locate a different therapist.
A good test is to scale back for a week or two and see how you feel. My guess is you'll be doing MUCH better. As long as you're up and walking around the house and maybe a little walking outside each day, there is no need to do a long list of therapy exercises. As I said, you have PLENTY of time once you're healed to build muscle strength. Your surgeon messed with bones and a lot of soft tissue during the surgery. It's irritated and it needs time to heal and settle down before you start pushing very much.