You don’t need to run DDU every time there’s an update. You only really need it if you’re having driver issues that can’t be fixed any other way or if you’re switching GPU to a different brand. And if you decide to run it make sure you do it in safe mode so it can actually remove everything.

It‘s the other way around. Capacity decreases on its own just through usage. What Apple (and other manufacturers, as you said) does is decrease clock speeds of the CPU and RAM to make degraded batteries last longer. Basically trading performance for battery life. And that feature should deactivate automatically if the device senses a new battery being put in. At least it did with my old iPhone 6S.

I use a plugin called Video Speed Controller, it‘s probably my most used browser extension. You can configure hotkeys as well, which makes it very convenient. I use Q/E for -0.5/+0.5 and A/D for -0.1/+0.1 so I can finetune the playback to my liking with just couple keypresses.

There’s also a small window in the top left of the video showing the current speed, which you can toggle by pressing V.