I'm also having a very similar issue with a T630. I know it's a different CPU (GX-420, 2.0 GHz base, 2.2 GHz turbo) but the same thing is happening. When under a single threaded load it will turbo up to 2.2 GHz, however when running a multi-threaded load each core will only reach about 1.4 - 1.6 GHz and the benchmark results are lower than expected: HP HP t630 Thin Client - Geekbench BrowserHi.
I'm fighting with cpu scalling on my fresh proxmox setup.
Whatever I try, my T730 never went beyond 1396MHz on cpu.
Not sure if this is a power/TDP issue for this platform, I've tried changing the power settings in the BIOS but it hasn't changed this. This same issue occurs on both Windows 10 and Ubuntu 19.04 that I've tried.