Large difference between elapsed and CPU times

[JimGolding]

What determines the difference between elapsed time and CPU time as measured by ⎕MONITOR?
I presume that three causes are external calls, paging, and context switching among users. Are there others? Do compactions, for instance, not count in CPU time?

The specific instance that I am looking at is
:If 0∊⍴ix←xWhere(↓pool.Data[;,pool.Flds⍳pREPDATAPOOLSRCEID],fcfld)∊↓translateb⌿fldm[;fldf⍳pREPDATAPOOLSRCEID,pPTRIK]
with <pool> being a namespace, its variable Data being a very large nested matrix, and <xWhere> is essentially ⍸.

Our internal profiling tool reports 5,256.87 seconds elapsed and 3,433.31 seconds CPU (for a large number of iterations). It’s possible that there were other users on the system at that time, but that is an awfully big difference.