Large in-memory replace becomes slow

#Idea
#Replace values
#Give a CSV with old value, new value
#Replace a single value
#
#



Function Update-OCSXML {
param(
    $BuddyListFile="A:\Export-Updated.xml",
    $NameMatchingFile="C:\Users\sowen\Dropbox\Docs\foxDeploy.com\Contoso\Contoso - Source Disabled Users with OCS Accounts - Disable OCS - 20141002.csv",
    $OutFile = ("A:\temp\" + (get-date -UFormat %Y%m%d_%H%M) + "_UpdatedOSCSXML.xml")
)

    begin{
        $i=0
        Write-host "Importing contents of `$BuddyListFile......" -NoNewline
        $FileInMemory = Get-Content $BuddyListFile
        Write-host "[OK]" -ForegroundColor Green
        $starttime = get-date
        if ($NameMatchingFile){
            Write-host "Importing contents of `$NameMatchingFile..." -NoNewline #I'm doing two Where's instead of one, this makes it faster somehow
            $NameMatchingFile = import-csv $NameMatchingFile | ? {$_."msRTCSIP-PrimaryUserAddress"}| ? {$_.mail} |select @{Name=‘Source‘;Expression={$_.'msRTCSIP-PrimaryUserAddress'}},@{Name=‘Target‘;Expression={$_.mail}}
            Write-host "[OK]" -ForegroundColor Green
        }
    #EndOfBeginning
    }
  process{

        if ($NameMatchingFile){
            ForEach ($name in $NameMatchingFile){
                $i++
                $nowTime = get-date
                $projectedTime = ($nowTime - $startTime).Seconds / $i * ($NameMatchingFile.Count - $i)
                if ($projectedTime -gt [math]::Abs(60)){$projectedText = ([math]::Round(($projectedTime / 60),2), " minutes" -join " ")}
                    ELSE{$projectedText = (([math]::Round($projectedTime,2))," seconds" -join " ")}


                $percent = [math]::Round(($i/($NameMatchingFile.count)*100),2)
                Write-Progress -Activity ("Processing record $i of " + $NameMatchingFile.Count + "| Projected time remaining: $projectedText") -PercentComplete $percent -Status "$percent`% Completed" -CurrentOperation ("Replacing " + $name.Source + " with " +$name.Target +"...")

                #Replace $nameA with $nameB
                $FileInMemory = $FileInMemory -replace $name.Source,$name.Target

                $lastTime = $nowTime
            }
        }
  #EndOfProcess
    }

     end{

     #Remove references to ContosoAd
        Write-host ("Replacing Contosoad.Contoso.com with Contoso.com..." ) -NoNewline
        $FileInMemory = $FileInMemory -replace 'Contosoad.Contoso.com','Contoso.com'
        Write-host "[OK]" -ForegroundColor Green

    "Outputting file $Outfile"
    $FileInMemory | Out-File $OutFile
    }

#EndOfFunction
}


Update-OCSXML