There is no such thing as "fast" in space; in space all speed is relative so "fast" always depends on what you are comparing against. There is no absolute point that stands still to measure speed against in space; even the sun moves (really fast!) around the galactic centre. Coupled mode tries to introduce a fake absolute speed, but it is just horribly contrived. In decoupled you fly like a real space ship. Absolute speed no longer exists, only your speed relative your target remains and there is no advantage to "high speed" because (fake) "high speed" simply vanishes. This is why good dogfighters always fly decoupled, it lets you fly relative your opponent so you don't need to compensate for your speed relative the sun all the time. Of course with MM, you hit that artificial speed-wall in space and boost very confusingly forces you to temporarily move relative the sun instead of relative your opponent. All that is artificially imposed by MM, not how speed works. The irony of it is that we have real atmospheric flight on planets and moons. Why do an unavoidably poor fake of atmospheric flight in space when we have the real thing? We just lose the interesting contrast to real space flight! Coupled mode is still useful. All it really does is "set speed relative chosen point". Right now the "set speed" function is limited to always be relative the sun/space stations/planets/etc, but it could easily be set relative whatever you target instead, making e.g. formation flying very easy. If your counterpart changes course or speed you will have to "set speed" again, but it is far more useful than the "always set speed relative sun" that we have now.