It is looking slow in the current videos we have been given access to, but this is still early days and all the materials are new. Not negating the fact this has been one of the coldest January's for some years, I think this performance, or lack of, is to be expected.
I saw someone also comment that they speculate about the possibility of H&S interfering with allowances for forces.
While there are limitations on the amount of sustained g-forces permitted on rides, they are far higher than the original Nemesis had.
It is also probably not possible to trim these down as the profiling of the track is the same, and gravity hasn't been on a diet recently, that I know of, and trim brakes have not been added anywhere on the layout. I also don't think increased friction wheel assemblies would be possible, because of, well, friction.
All rides are fairly slow when they're new and over time, with track and bearing wear, and warmer weather, bed in.