In a network based on packet switching, processing delay is the time it takes routers to process the packet header.
Processing delays in high-speed routers are typically on the order of microseconds or less.
However, in some systems, the processing delay can be quite large especially where routers are performing complex encryption algorithms and examining or modifying packet content.
[1] Deep packet inspection done by some networks examine packet content for security, legal, or other reasons, which can cause very large delay and thus is only done at selected inspection points.
Routers performing network address translation also have higher than normal processing delay because those routers need to examine and modify both incoming and outgoing packets.