Andrea,
I can tell you that 2.4.0 is as fast as any other raw processor for the Mac, and I pretty much have them all. System 10.13.5 beta with 16GB ram.
There is a flatpak of darktable as well, available from flathub.
Some time ago I did a formula for homebrew (I receive a lot of help from Darktable team). This formula was not accepted by homebrew team for different reasons.
The next week Iāll verify if it works. Itāll allow to build darktable easily for eager mac users.
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As a follow-up, in the end I didnāt get the iMac, but a Dell Inspiron 7775, a 27" all-in-one with 4k display with Ryzen 7 and AMD Radeon RX580. Darktable works wonderfully well on it 
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