What’s the difference between a M4 max and a “real” desktop processor?

It can’t be used as a space heater?

It’s not that it’s worse than a “real” desktop chip. In a way it’s better you get almost comparable performance with way lower power usage.

Also the M4 Max has worse MT performance than e.g. the 14900k which is architecture ancient in relative terms and also costs a fraction

Generally PCI-E lanes and memory bandwidth tend to be the big difference between mobile and proper desktop workstation processors.

Core count used to be a big difference but the ARM Procs in the Apple machines certainly meet the lower end workstation parts now. to exceed it you're spending big big money to get high core counts in the x86 space.

Proper desktop processors have lots and lots of PCI-E Lanes. The current cream of the crop Threadripper Pro 9000 series have 128 PCI-E 5.0 Lanes. A frankly enormous amount of fast connectivity.

M2 Ultra, the current closest workstation processor in Apple's lineup (at least in a comparable form factor in the Mac Pro) has 32 lanes of PCI-E 4.0 connectivity that's enhanced by being slotted into a PCI-E Switch fabric on their Mac Pro. (this I suspect is actually why there hasn't been a rework of the Mac Pro to use M3 Ultra - that they'll ditch the switch fabric for direct wiring on their next one)

Memory bandwidth is a closer thing to call here - using the Threadripper pro 9000 series as an example we have 8 channels of 6400MT/s DDR5 ECC. According to kingston the bus width of DDR5 is 64b so that'll get us ((6400 * 64)/8) = 51,200MB/s per channel; or 409.6 GB/s when all 8 channels are loaded.

On the M4 Max the reported bandwidth is 546 GB/s - but i'm not so certain how this is calculated as the maths doesn't quite stack up from the information i have (8533 MT/s, bus width of 64b, seems to point towards 68,264MB/s per channel. the reported speed doesn't neatly slot into those numbers).

In short the memory bandwidth bonus workstation processors traditionally have is met by the M4 Max, but PCI-E Extensibility is not.

In the mac world though that's usually not a problem as you're not able to load up a Mac Pro with a bunch of RTX Pro 6000s and have it be usable in MacOS. You can however load your machine with some high bandwidth NICs or HBAs i suppose (but i've not seen what's available for this platform)