FWIW, someone opened a bug report
here and Kovid found an elegant solution
here.
Which means calibre v2.36.0 and on will automatically have that (faster) workaround. That may not help you, stuck as you are on v1.48.0 -- but it should help the other people in that thread.
EDIT: Also, obviously, that fix can be applied
instead, which might be more efficient for you too.