Actually if you use the smm verifier that's been cut down to about 800ms. :) However, most of the theorem provers of today have been built on the philosophy that performance doesn't matter, or at least is secondary to ease of use, mathematical cleanliness etc, from the functional programming community. It turns out that once you make this decision it's difficult to get that raw speed back, even if you start worrying about performance later, and the HOL family provers make it worse by defining correctness in terms of the running of an ML program, which bakes the runtime of the ML system into the proof checking time.

When you combine this with the fact that these ML programs are not proofs but proof scripts, that perform a lot of "unnecessary" work like searching for a proof rather than just going straight for the answer, it suddenly begins to make sense why these systems take on the order of hours to days to check their whole libraries.

Coq and Lean are somewhere in the middle, because they have proof terms, but the logic itself still requires some unbounded computations. Checking a proof here is often fast, unless you make too much use of computation in the logic. But people often don't care about proof terms, and still store the proof scripts, which are as slow as ever.

Metamath is in this setting somewhat unique in eschewing proof scripts altogether, or more accurately, inlining proof scripts immediately on the author's machine. The resulting proofs are often comparatively long and verbose, but I would argue this is only a display matter, since all the other provers are doing the same thing, they just aren't showing it.