Yes, I am experiencing the same thing: slow data transfer from within a guest on VMWare Workstation 12.5.6 when writing to USB3 attached drives connected to the Win10 Host.
I have been working with this USB3 setup under VMWare about 6 months. (In my case the guest OS is FreeNAS).
I notice that the guest does detect the USB3 devices and reports them at 400Mbs devices, but actual transfer rate is much lower--more like USB2 speeds. I also notice that the CPU (Intel Core i5-6500T) is fairly taxed during transfers (85% or more, with nothing else running).
Prior to 6 months ago, I couldn't make USB3 devices work at all in this setup. Subsequent updates (mostly to the guest OS) improved things considerable.
It seems to me like USB3 support is still new, is difficult to make work (even on native hardware without VMWare in the mix), and that USB3 is now working on VMWare--but does put a lot of load on the CPU.
I am hopping that future updates to VMWare Workstation will improve performance of USB3 transfers, but there may be practical limits to the maximum transfer performance possible in a virtual machine.
