More efficient PowerShell query for MA Audit Drop File XML query

I am working on a variation of the Safety Catch script for a customer whereby I have an MA (sync) filter specified in the application settings of MIISERVER.EXE.CONFIG file, and I need to exclude CS objects that are filtered disconnectors as a result. The following is my code - which works OK, but is kind of slow, and I am after some ideas as to how to do this more efficiently. Any ideas please?

The following code snippet is a variant of the "Get-ThresholdCounters" function, and uses a new function to return the XML from miiserver.exe.config.

$FIMServicePath = "E:\Program Files\Microsoft Forefront Identity Manager\2010\Synchronization Service\"

    [xml]$directoryEntries = Get-AuditDropFile -FIMServicePath $FIMServicePath
    $ns = New-Object Xml.XmlNamespaceManager($directoryEntries.NameTable)
    $ns.AddNamespace( "a", $directoryEntries.DocumentElement.xmlns )


    # Examine delta data
    $Counters.StepType = $directoryEntries.mmsml."step-type"
    $deltas = $null #$directoryEntries.SelectNodes("//a:delta",$ns)


    # Client only - load miiserver.exe.config files to read config data
    [xml]$fimConfig = Get-FIMSyncConfigFile -FIMServicePath $FIMServicePath
    $sourceContactDNsToJoin = @($fimConfig.configuration.applicationSettings.'Mms_ManagementAgent_CORPExtensions.Properties.Settings'.setting.Where({$_.name -eq "sourceContactDNsToJoin"}).value.ArrayOfString.string)


    if ($sourceContactDNsToJoin) {
        # Client only
        $deltasToRemove = @()
        foreach($dn in $sourceContactDNsToJoin) {
            $deltasToRemove += $directoryEntries.SelectNodes("//a:delta",$ns).Where({$_.dn -like "*$dn"}).dn
        }
        $deltas = $directoryEntries.SelectNodes("//a:delta",$ns).Where({$_.dn -notin $deltasToRemove})
    } else {
        $deltas = $directoryEntries.SelectNodes("//a:delta",$ns)
    }

Any help most appreciated.