What is "Unaccounted Wait"?
Unaccounted Wait means that the PHP process is waiting for I/O, however the amount of time cannot be accounted for by data from the timeline trace, such as SQL queries or HTTP calls.
This is often the case when the server is under high load, and the PHP process executing the request is not getting resources from the CPU to execute, effectively sleeping / waiting for the CPU. It can also be the case when Tideways is missing an instrumentation for a database, cache, or any other kind of service you talk to over I/O.
Unaccounted Wait is calculated using the following formula:
I/O = (Wall-Time - CPU Time) - (Known I/O operations e.g., SQL, Memcache, File I/O)