Manual:$wgProfileOnly

Profiling: $wgProfileOnly
Don't put non-profiling info into log file
Introduced in version:pre 1.1.0
Deprecated in version:1.23.0 (Gerrit change 121336; git #d966a526)
Removed in version:1.35.0 (Gerrit change 560355; git #049dd15d)
Allowed values:(boolean)
Default value:false

DetailsEdit

Don't put non-profiling info into log file

Since MediaWiki 1.23, you can set the log file in $wgDebugLogGroups ['profileoutput'] to achieve the same functionality.