Before adding any progress indicator I think you'd do yourself a huge favor by switching to the Get-WinEvent cmdlet and let the target system do the search instead of having Get-EventLog return a huge number of events to be filtered locally.
Get-WinEvent -FilterHashtable @{LogName="Security";ID=4625} -ComputerName $server
Here's one link that uses the same criteria as your example: better-event-logs-with-powershell
If all you want as a progress indicator then pipe the results into a ForEach-Object and use "Write-Host '.' -NoNewline" followed by whatever data extraction you'd like.