Powershell Speed: How to speed up ForEach-Object MD5/hash check

As suggested in the comments, you might consider to start hashing files only if there is a match with the file length found first. Meaning that you will not invoke the expensive hash method for any file length that is unique.

_{*Note: that the Write-Host command is quite expensive by itself, therefore I would not display every iteration (Write-Host $lineCheck) but e.g. only when a match is found.}

$Folder = Read-Host -Prompt 'Enter a folder path'

$FilesBySize = @{}
$FilesByHash = @{}

Function MatchHash([String]$FullName) {
    $Hash = (Get-FileHash -LiteralPath $FullName -Algorithm MD5).Hash
    $Found = $FilesByHash.Contains($Hash)
    If ($Found) {$Null = $FilesByHash[$Hash].Add($FullName)}
    Else {$FilesByHash[$Hash] = [System.Collections.ArrayList]@($FullName)}
    $Found
}

Get-ChildItem $Folder -Recurse | Where-Object -Not PSIsContainer | ForEach-Object {
    $Files = $FilesBySize[$_.Length]
    If ($Files) {
        If ($Files.Count -eq 1) {$Null = MatchHash $Files[0]}
        If ($Files.Count -ge 1) {If (MatchHash $_) {Write-Host 'Found match:' $_.FullName}}
        $Null = $FilesBySize[$_.Length].Add($_.FullName)
    } Else {
        $FilesBySize[$_.Length] = [System.Collections.ArrayList]@($_.FullName)
    }
}

Display the found duplicates:

ForEach($Hash in $FilesByHash.GetEnumerator()) {
    If ($Hash.Value.Count -gt 1) {
        Write-Host 'Hash:' $Hash.Name
        ForEach ($File in $Hash.Value) {
            Write-Host 'File:' $File
        }
    }
}