The processors are underclocked, probably for heat considerations. The Turbo Boost mitigates some of that (they reduced the boost only 300 MHz on the 8-core, and not at all on other models).
One interesting loss is that the 8-core, which got hit by 500 MHz in base frequency, will almost certainly be slower than a stock-speed Coffee Lake i7-8700K on most tasks. The underclocked Xeon W-2145 in the base iMac Pro is 8 cores, 3.2 gHz, turbo to 4.2. The 8700K is 6 cores, 3.7 gHz, turbo to 4.7, and it's 2 generations newer in architecture (Coffee Lake versus a variant of Skylake).
Under most circumstances, you'd rather have 500 MHz on both base and turbo clocks (about 12%) on a newer processor design than 25% more cores. There are probably some applications that are so heavily multithreaded that they'd prefer the cores, but there aren't many, and the difference will be small (even perfect use of all cores, which only a benchmark can do, should leave the Xeon only about 10% faster). The Xeon also has an advantage in memory bandwidth that will help in some cases.
Most photographic applications are optimized for up to four cores, plus it's nice to have a couple of cores if you're running Mail, Safari, Word, InDesign or whatever in the background. Six cores help because of background applications, but eight are hard to use in photography right now.
Why am I focused on the 8700K? Because it's probably what's in the next (non-Pro) iMac! Unless Apple underclocks it, too, the fastest standard iMac will soon beat the 8-core iMac Pro in almost all photographic applications. Even if Adobe, Phase One and everybody else rewrite the software to use cores efficiently, the low-end iMac Pro will only be ~10% faster than a garden-variety iMac in a few months.
If photo software starts using a ton of cores efficiently, the higher core count iMac Pros become more interesting, and the processor upgrades are not outrageously priced, given what the Xeons actually cost.
With current software, though - wait for a 6-core iMac (or even a 6-core MacBook Pro if they solve the 16 GB RAM limit) - it'll be just as fast and a lot cheaper. The exception is software that either uses the GPU really efficiently (who knows what will be in that 6-core iMac, but it might not be close to the Vega in the iMac Pro), uses cores efficiently (get a 10,14 or 18-core iMac Pro) or hits memory bandwidth REALLY hard.