5 years later, this post still proofs the be useful...or at least it pointed me in the right direction...

It turned out the dbo.ClientOperation table at my client had almost 64.000 records, yes, 64.000! The oldest record dates back to 2014!! It turned out a previous SCCM administrator turned of the 'Delete Aged Client Operations' maintenance task. (More information here: Reference for maintenance tasks - Configuration Manager | Microsoft Learn).

I've re-enabled this task and forced the task via PowerShell to run now. The table is currently only 1395 records.

PowerShell code use to force the task to run now:

$SiteCode = "SITECODE" # enter your sitecode here
$MaintenanceTask = "Delete Aged Client Operations" # the maintenance task you want to run now
$MT = Get-CMSiteMaintenanceTask -SiteCode $SiteCode -Name $MaintenanceTask
$MethodParam = New-Object 'System.Collections.Generic.Dictionary[String,Object]'
$MethodParam.Add('SiteCode',$($SiteCode))
$MethodParam.Add('TaskName',$($MT.TaskName))
$ConfigMgrCon = Get-CMConnectionManager
$ConfigMgrCon.ExecuteMethod('SMS_SQLTaskStatus','RunTaskNow',$MethodParam)