Zwift has noted that they’ve very conservatively tuned the grades to be much easier than the display suggests. 10% feels more like 3%. Hopefully they’ll reconsider this when adding new routes.
There’s no way to really do tough hills yet unless you adjust your gears and drop cadence. The problem with that now, though, is that Zwift will still record/display you as going much faster.
I guess they didn’t want to turn casual riders off too much at this stage by presenting a hill they might not be able to handle when using a smart trainer. They could have just tuned everything more accurately and only had grades under 3%, but then the visuals would be flat and less dramatic.