While I don't know how bad the performance would be on huge repos, there is of c...

C4K3 on Aug 16, 2016 | parent | context | favorite | on: Setup Keybase.io, GPG and Git to sign commits on G...

While I don't know how bad the performance would be on huge repos, there is of course an option to turn it on or off (--[no-]verify-signatures.) which I believe is off by default.