Truthfully though you're right, using typical linkers, this would be pretty slow; at least a few seconds for large binaries, to minutes for things as large as web browsers. However, for many binaries, linking can be done much faster; mold claims to be only 50% the runtime of using `cp` on the object files, which is fast enough to even re-link Firefox on-the-fly without it being unusable.

You could imagine writing a linker specifically for this case, that encodes information about the object files directly into the resulting bundle.