What is "Unaccounted Wait"?
Unaccounted Wait means, the PHP process is waiting for I/O, but the amount of time can not be accounted 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.
To analyse the case, we recommended to trigger a trace with Trigger Trace or Boosting Traces to get a call-graph.
Unaccounted Wait is calculated as
I/O = (Wall-Time - CPU Time) - (known I/O operations like SQL, Memcache, File I/O).