The most interesting part, to me, was that entering SMM pauses all cores at once, instead of doing the work in a single core like normal interrupts. That sounds like a performance killer, and I hope entering SMM is really rare in modern systems.
My information is pretty out of date, but when TPMs first arrived on the scene there was a fair bit of talk about using them as secure enclaves where you could do honest to god "trusted computing" with a fully verified stack on ordinary PC hardware. This largely didn't work out because TPMs were slow and every time you tried to do it you basically stalled out the rest of the machine, so once execution came back to the CPU everything was out of sync and all of the attached hardware like network cards and video cards crashed or froze. TPMs ended up only being useful as a place to store keys and occasionally cryptographically sign small amounts of data.
That said, the SMM can probably be a little less intrusive if it needs to be. Like it doesn't have to freeze the cores if all it is doing is reading your bitcoin addresses and passphrase out of memory, just stalling the memory bus for a moment or two.
