How do exchanges send transactions efficiently?

Question

This site says that " (exchanges do not attach bitcoin miner fees)...Bitcoin exchanges often try to send their transactions as efficiently as possible. This can mean that the fee is not large enough or doesn’t arrive quickly enough for a miner to include your transaction in the next block."

What does it mean by "send their transactions as efficiently"? Do they just send transactions without attaching fees? Somehow the latter statements, which imply that exchanges do attach bitcoin miner fee, contradict with the content in the earlier bracket.

Answer 1

This can mean that the fee is not large enough...

This is saying that exchanges try not to overpay on fees, which might cause a delay in case the fee market situation changes abruptly – for example if the next block doesn't get mined for long enough that the transactions arriving afterwards push the exchange's transaction away from the top of the mempool. And as far as I know, exchanges generally don't bump fees using RBF.

...or doesn’t arrive quickly enough for a miner to include your transaction in the next block.

Exchanges usually batch multiple payments together into one larger transaction. Since a single-recipient transaction usually has one input and two outputs (one for the recipient and one change output), adding more recipients saves 66% in blockchain space (and therefore fees) in the limit. However, this also means the exchange won't send you your bitcoin right away, as they need to wait for other people to batch all your withdrawals together, and during this time they might miss a new mined block.