Can i provide a contrary opinion? I have Ventress at 7* and she is excellent at GW. More importantly, I have her in my Arena team and I got to no. 1 yesterday. The trick is to get around her speed and exploit all the other stuff she has going for her. Thus, my team is Phasma (L), Poe, GS, Lumi and Ventress. Phasma is key because of the turn meter manipulation and bonus attacks, which help Ventress attack more and out of her turn. Poe likewise gives her the chance to catch up. Also, each kill gives her a chance to gain turn meter. GS can call her for a fairly strong assist hit. And her buff removal/heal can also remove turn meter.
In a field with plenty of taunts, offense up and other buffs, particularly with Yoda around, she can turn the tide all of sudden. Let's also not forget her life steal, the highest in the game, which keeps her in the fight longer and heals her with an Aoe to full.
Speed will always be important and Ventress will always have that weakness. Maybe my team is a bit unique in that regard. But my point is that it can be overcome and used to your advantage.