The Merge Rigs button changed slightly in the latest version. Now all meshes are put in a separate sub-collection under the armature's collection, like this:
As a test, I imported a directory with 78 pose files. It took 32 seconds with the meshes in the scene, but only 2 seconds with the meshes excluded, i.e. a speed-up with a factor of 16. Not bad!
To get that speed-up, the meshes must belong to a collection which is excluded from the scene. Just hiding meshes in the viewport does not improve loading speed. Apparently hidden meshes are still updated.