Setting "Use special effect at reaching ground position": false does not disable the rocket for me.
The "signal rocket speed" setting also does not seem to work.

Am I doing something wrong ? Could it be a plugin conflict ? or is there an error in the plugin ?

Actually, I just realized that Supplysignal/droptypes has its own setting for "firesignalrocket", I kinda missed it since it's called "special effect" for default airdrops.
So it works fine when disabling that, my bad.

The SignalRocketSpeed option still does not seems to have any effect on the rocket speed.