Just a quick off the cuff and building off of what Dangph started but the triple scan is probably killing performance.

$Scriptdone = $Validate | Where-Object {$_ -match $Scriptcompletedsuccess}
$Updatedone = $Validate | where-object {$_ -match $Updatecomplete}
$Failed = $Validate | Where-Object {$_ -match $FailedValidaton}

Each one is reading through the entire file to find one thing. One route you can try out is using a foreach(You can shorthand it with % as I'll do below) and a switch.

$size = $array.Length
#Assumes there's only one line that will match a given regex per file
#If not, it'll add duplicates which can be stripped at the end with an $array | sort -unique
$validate |% {
    switch -regex($_){
        $Scriptcompletedsuccess {$array += "$IDNumber, $Good1"; break}
        $updatecomplete {$array += "$IDNumber, $Good2"; break}
        $Failedvalidation {$array += "$IDNumber, $Fail1"; break}
        default {}
    }

#Checks to see if array has grown, if it hasn't, no matches were found
#Bit hacky and there's probably a better way to do it.
if($size -eq $array.length){
    $array += -join ("$IDNumber",', ',"$Fail2")
}

Just a quick off the cuff and building off of what Dangph started but the triple scan is probably killing performance.

$Scriptdone = $Validate | Where-Object {$_ -match $Scriptcompletedsuccess}
$Updatedone = $Validate | where-object {$_ -match $Updatecomplete}
$Failed = $Validate | Where-Object {$_ -match $FailedValidaton}

Each one is reading through the entire file to find one thing. One route you can try out is using a foreach(You can shorthand it with % as I'll do below) and a switch.

$size = $array.Length
#Assumes there's only one line that will match a given regex per file
#If not, it'll add duplicates which can be stripped at the end with an $array | sort -unique
$validate |% {
    switch -regex($_){
        $Scriptcompletedsuccess {$array += "$IDNumber, $Good1"; break}
        $updatecomplete {$array += "$IDNumber, $Good2"; break}
        $Failedvalidation {$array += "$IDNumber, $Fail1"; break}
        default {}
    }

#Checks to see if array has grown, if it hasn't, no matches were found
#Bit hacky and there's probably a better way to do it.
if($size -eq $array.length){
    $array += -join ("$IDNumber",', ',"$Fail2")
}

Oh yeah, another performance boost (not sure how much of one though) would be changing the array into an ArrayList. They can append whereas the array resizes on each add.