HP 5500 switch manual states SNMP traps impact performance and advises against their use unless "necessary" - why?

Traps trade consuming memory and processing resources on the switch for reduced network usage.

1. Counters, status registers etc on network gear
2. Polling and tracking the above
3. Taking actions

Doing (2) and part of (3) on the switch means less network traffic querying SNMP MIBs. This would be a win if you have one piece of network gear at a remote location over an expensive link and you want to monitor things that don't trigger actions often.

For a local network a few traps for things that rarely trigger events can be a win as well when the underlying counter or registers are not already being queried for other purposes.