LND #2759 lowers the default CLTV delta for all channels from 144 blocks (about 24.0 hours) to 40 blocks (about 6.7 hours). When Alice wants to pay Zed through a series of routing nodes, she starts by giving money to Bob under the terms that either Alice can take it back after (say) 400 blocks or Bob can claim the money before then if he can provide the preimage for a particular hash (the key that opens a hashlock). The 400 block delay is enforced onchain if necessary using
OP_CHECKLOCKTIMEVERIFY (CLTV). Bob then sends the money (minus his routing fee) to Charlie with similar terms except that the CLTV value is reduced from Alice’s original 400 blocks by the CLTV delta of his channel with Charlie, reducing the value to 360 blocks. This ensures that if Charlie waits the maximum time to fulfil his HTLC to Bob and claim his payment (360 blocks), Bob still has 40 blocks to claim his payment from Alice by fulfilling the original HTLC. If Bob’s HTLC expiry time with Charlie wasn’t reduced at all and used a 400 block delay, Bob would be at risk of losing money. Charlie could delay fulfilling his HTLC until 400 blocks, and Alice could then cancel her HTLC with Bob before Bob had time to fulfil the HTLC.
Subsequent routers each successively subtract their delta from the value of the terms they give to the next node in the route. Using a high CLTV delta therefore reduces the possible number of hops that can be used in a route, and makes a channel less attractive for use when routing payments.