The problem isnt one thing. It is multifaceted. Yes speed is always king, but in some situations that can be overcome. Faster JTR? Use a high tenacity/crit avoidance team. Faster ewoks? AOE daze or a tank who counters and applies daze. Even Jedi revan, I'd take a slightly slower revan with a much stronger jolee/GK and higher offense on Yoda instead of speed.
Part of the problem is the current meta. Due to undispellable debuffs, AOE turn lockouts, and high damage/max health based damage. Missing a turn because of being slower means an auto loss.
Any fix would need to be multifaceted as well. Nerfing speed would just keep the same speed meta issues at lower top speeds.
The set bonus issue is just asinine. At the top end of nature shards you won't survive without a high speed speed set. It would just make everyone farm speed sets and health sets only and make all other mod sets useless.
It needs to be a much more comprehensive adjustment.