Do checkpoints affect initial chain sync time? I've seen some guides that suggest that initial sync time for a new full node can be accelerated by setting checkpoints: false but I don't understand why that would be the case. My thinking is the opposite would be true, because bitcoind won't bother fully verifying blocks below checkpoint height?
A Checkpoint is the header hash permitted at a given height, thereby hardcoding the chain that the validating node must follow. If a node follows an alternative chain branch which reaches the same height, this branch can now be identified as such and rejected. This was initially designed to protect a node from following an alternative long branch, especially generated with low difficulty, which could be so long that it would challenge the available resources of the validating node.

Validation used to be skipped prior to checkpoints, but that is no longer the case. Validation is skipped up to "known blocks", but only if these are found in the strongest chain. Today, checkpoints don't reduce chain sync time.