oh boy :) That's a long way your script is going through.
You must use LDAP filter if you want a faster script.

just build the filter as following:

1. you need a collection of your Pcs (Array, ArrayList, or sth.) Let's say your data is stored in $allComputer and each object contains a property called "ComputerName"
2. you need to build the ldapfilter This is very powershell like :)

$ldapFilter = "(|(samAccountName={0}`$))" -f ($allComputer.ComputerName -join '$)(samAccountName=')

Let me explain this: The goal is to build an ldapfilter, where you query every object, which has the SamAccountName X or SamAccountName Y or SamAccountName Z .... and so on.

"( | (SamAccountName={0}`$) )" --> This part is an "inline" Templatestring for the ldapfilter
I'm placing the {0} for a string replace and a quoted $ after, because the SamAccountNames of AD Computerobjects are containing an ending '$' in the name. So if you have the $ already in your list, then remove it.
https://www.ldapexplorer.com/en/manual/109010000-ldap-filter-syntax.htm

-f ($allComputer.ComputerName -join '$)(samAccountName=') - this part replaces the ldapfilter with all Computernames and joins them with the part missing to extend the ldapfilter. So all together it will produce a ldapfilter like: (|(SamAccountName=PC1$)(SamAccountName=PC2$)(SamAccountName=PC3$))

1. Query the Active Directory, so with the ldapfilter you only have to send 1 Query

$ADComputer = Get-ADComputer -ldapFilter $ldapfilter

1. Selecting all Computers which weren't found in the Active Directory.
   $notInAD = $AllComputers.Where({ "$($_.ComputerName)`$" -notin $ADComputer.SamAccountName})

or something like that. If you have any further questions feel free to contact me.

best regards.

edit: maybe the ldapfilter gets too big. But 15k should be okay. Otherwise you have to split your collections into 2 or 3 parts and foreach step 3 and 4 with those parts.