I recently did a test on taranis to find out what had the lowest latency with different radio firmware and protocols. Some of that was from this thread on opentx https://github.com/opentx/opentx/issues/4640
Radio firmware: Opentx vs Ersky9x
Protocol: SBUS vs Fport vs Spi (vs Multi module)
Video of my test setup and results:
Google Doc latency data: https://docs.google.com/spreadsheets/d/1aTRuUzEIAqn2h_WNNHb7oMXqKZB5olL95rLlrjfThHw/edit?usp=sharing
Need one of the recent ersky9x versions here and check double rates in the menu:
The results in best to worst:
Opentx frsky_x to x4r using sbus: 23-41ms
Opentx frsky_x to x4r using fport: 18-37ms
Opentx frsky_x to spi: 16-35ms
Ersky9x frsky_x to x4r using sbus: 17-28ms
Ersky9x frsky_x to x4r using fport: 13-24ms
Ersky9x frsky_x to spi: 11-22ms
The more limited testing with frsky_d and multimodule was worse then opentx/ersky9x sbus.
I have have swapped from opentx to x4r using sbus and to Ersky9x frsky_x to x4r using fport or spi on all my quads. That cuts the latency in almost 1/2 with only firmware changes!