It's going to be super fast. 32 core base model with available 64 core upgrade.
That means nothing without supported software. For instance in the scientific area, which always has been a major part of the Mac Pro's target audience, there are no high-performance programs at all which run on ARM CPUs. Intel, yes. SPARC, yes. Power (and PowerPC before that), yes. ARM? Zero. Many applications couldn't even be ported to ARM without massive drawbacks as they are built to make use of certain CPU-specific technologies.