I can't get my rear derailleur to downshift properly. Especially in the middle of the cassette it'll delay a bit before shifting.
Limits are set correctly.
Since I can't explain this properly I recorded a video
My only guess is it being slightly bent.
When downshifting the spring of the derailleur is pulling the chain off of the one gear. In contrast when upshifting the tension of the cable is causing the movement and there generally is much more force (tension on the cable). This is why upshifts typically happen faster.
If the downshift is accurate but slow then friction is usually to blame.
In addition to cleaning and lubricating the cable you can also try:
Checking the B-screw to make sure the jockey pulley isn't too far from the cassette cogs.
Turn the adjusting barrel clockwise (screw it in) a half turn. This will bias the rear derailleur towards the downshift. If you find the upshifts are getting slow or noisy then turn the adjusting barrel counter-clockwise a quarter turn.
This looks like a classic case of the cable sticking in the housing momentarily as the shifter lets out cable to downshift.
Your indexing looks fine as the chain is not rattling or trying to shift on it's own. FYI the derailleur high and low limits do not affect shifting or indexing.
Try removing the cable, wiping it down with a clean rag, applying a little chain lube and reinstalling. If that does not work replace the cable and housing. One cable and a length of housing is not expensive.
