I have started doing ftp workouts training plan and experienced quite frustrating (well frustrating for me) little bug. You ride nicely at prescribed wattage at recovery phase i.e. 90W and you hit a hill. Even 5% hill can kill your motivation but today I was cycling 90watts uphill with 11% gradient. Guess what? 2km/hr average speed. I was just about to give up. It would be good to keep min speed of say 10 or 15 km/hr so you can enjoy the riding and the views.
Your speed within Zwift should be based on real world, so if you are going up a 11% hill doing 90 watts you SHOULD be going slow. A workout is about watts over time, not in game virtual speed.
Make sure when doing workouts that you choose a flat route, your speeds won’t drop as much.