copydatabase glass->glass performance curiosity

While migrating some DBs from chert-to-glass with copydatabase(1),
I noticed copying from chert-to-glass was 2-3x faster than copying
from glass-to-glass.

Not that I have reason to copy glass-to-glass, but was testing
something and did not have an easy way to create chert DBs
from the Perl bindings.

Same SSD, with fstrim(8) issued in-between runs.

glass is way faster in general search performance, though :>