At startup, InnoDB is creating a redo log checkpoint.
This is not only slowing down the InnoDB startup. It is also potentially hiding crash recovery bugs, because crash recovery is only processing the redo log records starting from the latest checkpoint.
If the checkpoint is removed from InnoDB in 10.2, at least the following tests fail: encryption.innodb-redo-badkey encryption.innodb_encryption
In 10.1, all encryption tests pass even after the code is removed, but this could also be because the tests are less extensive in 10.1 than in 10.2.
The code was originally added in
MDEV-8041, apparently to work around fundamental problems in redo log encryption key rotation, which was finally disabled in MDEV-9422. ( MDEV-12041 was filed for re-enabling it.)