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.
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.