Ignoring your housing
Derailleurs get all the glory, but they are really a secondary contributor to shift performance. The shifters themselves are the key component, and they can only actuate the derailleurs properly if the connection between the two—the cables and housing—is functioning smoothly.
Road gunk gradually accumulates inside the housing and gums up the cables. Once that happens, no component group in the world no matter how expensive can shift crisply.
It isn’t a sexy upgrade, but regularly swapping old cables and housing for a new set is one of the best investments you can make to improve your component function. Swap housing about once a year, and replace the cables twice.