speed up in DC's scanning - multithreading

Question

I'm building a script which will go to each DC's and take the value from modifyTimeStampe values and then on take the maximum value as the following code, at the moment the code is too slow because now it goes to each domain and put the value to the array, then get the max value from the array out. I would like to speed it up. I was thinking to use multiple threads, but still looking for a better idea to implement.
$Searcher = New-Object System.DirectoryServices.DirectorySearcher
$Searcher.PageSize = 100
$Searcher.SearchScope = "subtree"
$Searcher.Filter = "(&(objectCategory=$objectCategory)(objectClass=$objectClass))"
$Searcher.PropertiesToLoad.Add("distinguishedName")|Out-Null
$Searcher.PropertiesToLoad.Add("modifyTimeStamp")|Out-Null
Function modiScan{
forEach ($users In $userObjects)
{
$DN = $users.Properties.Item("distinguishedName")[0]
$dnarr.add($DN)|Out-Null
}
#$dnarr
foreach($dnn in $dnarr){
$error = $false
$lastmd = New-Object System.Collections.ArrayList
ForEach ($DC In $Domain.DomainControllers){
$Server = $DC.Name
$Base = "LDAP://$Server/"+$dnn
$Searcher.SearchRoot = $Base
try{
$Results2 = $Searcher.FindAll()
ForEach ($Result2 In $Results2)
{
$DN2 = $Result2.Properties.Item("distinguishedName")[0]
if($DN2 -eq $dnn){
$modi = $Result2.Properties.Item("modifyTimeStamp")[0]
if($modi){
$lastmd.Add($modi)|Out-Null
}
}
}
}
catch{
$error = $true
}
}
if($error -eq $true){
$lastModi = "None-set"
$global:noneModi++
}
else{
$lastModi = ($lastmd |measure -max).maximum
if($lastModi -ne $null){
$lastModi = $lastModi.ToString("yyyy/MM/dd")
if($lastModi.split("/")[0] -eq 2015){
$global:modi2015++
}
elseif($lastModi.split("/")[0] -eq 2016){
$global:modi2016++
}
elseif($lastModi.split("/")[0] -eq 2017){
$global:modi2017++
}
else{
$global:otherModi++
}
}
else{
$lastModi = "N/A"
$global:noneModi++
}
}
#$lastModi
$obj = New-Object -TypeName psobject
$obj | Add-Member -MemberType NoteProperty -Name "modi" -Value $lastModi
$obj | Export-Csv -Path "$outFileModi" -NoTypeInformation -append -Delimiter $Delimiter
}
}
modiScan