git takes ages after fetching ports in the resolving deltas stage

Hello arm@,

Lately I'm finding that git is taking up to 10 minutes in the 'resolving deltas' 
stage when fetching ports.

I'm thinking maybe the git parameters I'm using are suboptimal. Mentioning this
here because the time taken is really apparent on relatively slow hardware 
like the rpi4. This machine uses 'git -C /usr/ports fetch --ff-only -q' to
refresh the local ports tree. Should it be using something else?

The ports tree was initially fetched with 
'git clone ssh://anonssh@git.freebsd.org/ports.git'
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