Constant MAX_CHECKPOINT_HEIGHT_GAP 

pub const MAX_CHECKPOINT_HEIGHT_GAP: usize = 400; // 400usize

We limit the maximum number of blocks in each checkpoint. Each block uses a constant amount of memory for the supporting data structures and futures.

We choose a checkpoint gap that allows us to verify one checkpoint for every ObtainTips or ExtendTips response.

zcashd’s maximum FindBlocks response size is 500 hashes. zebrad uses 1 hash to verify the tip, and discards 1-2 hashes to work around zcashd bugs. So the most efficient gap is slightly less than 500 blocks.