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:
http://openrcforums.com/forum/viewtopic.php?f=7&t=4676#p65894

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!