Any idea whats going on here ?
Of course, it's very simple. You aren't running Mac :- ).
(No idea, my wager would be the difference is due to BLCK multiplier)
Now to this interesting discussion we have about merits of Windows vs Mac machines. I will go point by point and later point out it is very Apple™ and Oranges but why not play the game nonetheless :- ) ?
1) "Hassle". This isn't due to Windows vs Mac nature but the nature of Bespoke (+home)-build vs Prebuilt Machine. You can buy prebuilt Windows based workstations just as well from Dell,IBM, HP, Boxx,etc. They will also feature partly (though not to an extent of current-gen iMac) custom sourced hardware that's guaranteed to work together as it was pre-tested. Downside is huge price premium, lack of repairability outside of official channels due to custom hardware (mostly mainboard + PSU, but often the case as well).
Issues mentioned in this thread are mostly multiplied by the niche nature of enthousiast platform that was rushed to market, made in small orders and wasn't paid enough attention by 3rd party vendors (particularly the business of selling motherboards is very thin margin so even if brands like MSI create 'Halo' product for niche platform, it doesn't mean they paid attention to it to an extent that corporate deployed platform would. Everyone going this route knows there are risk involved. Risks with big payoff in price and performance.
I can (and have done so for family members and friends) build simple PC/Workstation on more mainstream platform (or pro-grade like Xeon) within hour and no complication ever after.
2) Price & Performance payoff. Now here I will show just how much the difference is actually. Because the aggregated Passmark score you have quoted is very misleading (it doesn't scale well and it's aggregate of lot of single-core processes, IPC has not grown much over the years. Basically, 8-core will load Chrome page just as fast as 32-core monster, so the scores will be similar), it basically doesn't even apply at all to Corona rendering. Better example is Cinebench R20, which perfectly scales in all situations.
Your configuration (iMac Pro, 8-core base model, 27" display) uses Xeon W-2140B from 2017. The multi-threaded score is
3700, rounded up.
Threadripper 2990WX has a score of
11 500+ at base-clock, and can go up to
13 000 with good cooling (like the workstation I have built for Veronika, on page 9?).
So it is 3 times faster. But that is expected because we are comparing and 8-core vs 32-core. Nothing to do with Windows or Mac, but lot to do with Threadripper vs Xeon pricing.
You absolutely can build 2990WX + 32GB + Vega 56 for the price of iMac pro minus monitor. Let's see the math (Amazon US prices):
Threadripper (2990WX=1622$, Aorus Xtreme 429$, 32GB DDR4 3000 CL15 179$, Fractal Case Define R6 149$, Noctua UH-14S TR3 79$, Vega 56 276$, 1TB Samsung 860 EVO 147$, 750W Seasonic Platinum 119$, 10GBE is on-board)
TOTAL:
2,999$ (wow heh) + 27" LG 5K Ultrafine (this the panel Apple uses and this is the display they sell separately) 869$ = Grand Total
3869$So we paid 30perc. less for 300perc. of the performance. And we have easily repairable, upgradable workstation with display we can swap or replace anytime.
To get closer to Threadripper 2990WX, we have to upgrade to 18-core version ( Xeon W-2191B ) but which has low TDP for cooling purposes so the all-core boost is just slightly above 2.3GHz, rendering the performance to be roughly half of 2990WX.
The price would jump to 7390$ Now the dirty equation would be half the price for twice the performance.
Of course, not all is fair above. iMac is all-in-one PC, it's made for creative professionals, but those creative professionals are meant more to be photographers, 2D artists, etc.. It's not rendering machine by any mean.
And there are Windows alternatives which are just as expensive and weak, for example Microsoft Surface Studio 2 which has all the same benefits and drawbacks like the Apple one but is arguably superior also but that is different topic altogether. Since you wrote this in Threadripper 2990WX thread, I compared the Apples&Oranges you brought :- ).