Going low is not all about bending at the knees but really more at the hips. Too much knee bending actually makes you slower. The knee bend actually doesnt lower your centre of gravity by much.
100%. I used to bend too much at the knees and it was a terrible idea. Footwork becomes slower and robs you of power in shots.
For explosive movements, it's better to load and then release energy in a short a time frame as possible. When people want to jump high, they aren't going down to full squat first. At most you do maybe a quater squat before jumping.
People always recommend keeping a low center of gravity but they never actually explain why it would be beneficial to have a low center of gravity at all times. There are a few obvious advantages. One is that shots are easier to judge at eye level. Another is that getting your whole upper body structure closer to the point of contact gives you a lot more control (not sure how much of that is merely from better vision or total proprioception). But staying low at all times seems like a bad idea from a footwork and power perspective.
I posted a video about the explosive "first step" and it argues that you gain speed by 'falling' towards the direction you want to travel. So you would need to have a higher stance to start with so that the potential energy can be used for movement.
This is obvious when observing fast players are on serve receive. They might set up low to get their eyes to net level, but they all end up standing up pretty straight when the serve actually happens, then drop back low again depending on what shot they play (e.g., going forward for a push/flick, or loading a FH/BH etc).
Staying low the entire time would really hinder your movement since you don't can't borrow any momentum from gravity or stored elastic energy in youry body (good article about it here:
https://protabletennis.net/content/elastic-energy-stroke-play). It's also too tiring for your legs since you'd basically be playing a game while in the horse stance.
So it's not about keeping a low center of gravity, but about starting from a balanced posture that lets you 'fall' towards whatever position you need to get to and/or store that power for your stroke. For a big FH, you fall towards a position that loads your right leg and hips, and then you release that power into the ball by pushing off the leg and rotating the hips. For a short push, you start high, fall forward for the touch and load the front leg, and then push off that leg to bounce back to a higher balanced posture again to be ready for the next shot.
This is probably the reason why 'adding' an additional recovery step seems to make people faster. For one, in the recovered position you now can use gravity to quicken your next move. But I suspect the major issue that if you don't do the recovery step, it means your momentum is often directed in the wrong direction in relation to next move you want make.
The "first step" and the storing of elastic energy concepts really helped round out my understanding of why the recovery step was a good idea. But the principles pretty much apply in every situation where you're moving your body around in space.
