Install htop or, if already there, top, and check that you aren’t CPU-bound. Don’t open LuCI while trying to push the router hard as single-core, MIPS-based SoCs, in general, don’t have a lot of CPU power and LuCI can consume quite a bit alone.

Is your client single-stream, or does it support multiple streams? Checking

iw dev <wlan device> station dump

should show the modulation type of the connection in use.