A user reported on gnupg-users a severe slowdown in --list-keys in 2.2.3 compared to 2.1.23. it might be due to a new key but may be relatesd to something elese. We need to get better performance figures to understand such slowdown.
Running tests with a modified keybox, so that the the keybox has only the meta data and the actual keyblocks are stored in separate files improved --list-keys by a factor of 10. This can be explained by reducing the size of pubring.kbx (which is sequentially scanned) from 95 to 2.5 MIB.
Another data point is that not using the signature cache has a more than 10-fold degradation on a simple -k.