It's a bit tough to say, but from trying to squeeze the best time possible out of the tree chopping challenge, I'd say my best rhythm there is around 2.5-3 hits per second. Try playing the
main game and get used to how long the various tool animations last. For some people it's easier to watch for the exact frame when to click in order to get the next hit immediately, without the character idling, for others it's better to go by the sound of the swing.
The highlighted trees don't become semi-transparent, only the not-targeted nearby trees do, that can make it a bit harder to chop a tree from above, since I can't see the animation and unless I position myself a bit to the side, I have rely entirely on sound to know when to hit again.