If shifting deteriorates after a fall onto the right side, the derailleur hanger may be bent. The derailleur pulleys then sit out of alignment with the cassette. Looking from behind can reveal an obvious lean.
Accurate checking uses a hanger alignment gauge, with the rear wheel first seated straight in the frame. Repeatedly bent hangers can break during correction. A workshop can assess alignment or replacement; derailleur settings must be checked again afterwards.
The hanger is not the only suspect. On mechanical drivetrains, inspect cable movement, housing and chain wear too. Contamination can create friction that delays shifts, while worn chains and sprockets can cause skipping under load.