When doing a resistance exercise, even a body-weight one do a rep differently. 10 body-weight squats may not seem that difficult and you can do a couple sets of those with ease. Perhaps you have a good hold on pull-ups, push-ups or negative push-ups. Next time try a 10-count until get to the bottom of that squat, then 10-count to the top. Eventually you may find yourself getting deeper into that squat & also noticing where the tension lies in your legs at a certain point. Just five slow reps will show you how you focus more on your form, holding it & of course looking forward to then end of that count! But seriously, pumping out a faster/your usual speed is one thing but fighting gravity both ways and feeling those muscles strain will give you an appreciation of a slower & tougher workout without adding any additional weight.
Obviously some exercises may lend themselves easier to going slow than others & not all gym machine equipment is really made for slow motion. Nautilus is a brand of equipment that is used for such workouts. Maybe ask at your gym if they have it.
Give it a shot – going slow is not easy, its lower impact and high intensity.
Here are some resources.
A local facility in Encinitas specializes in this. I went to an introductory session there & it really opened my eyes to this type of strength training.