Help with querying a large number of Active Directory accounts (PowerShell)

Just do the first method. This won't even put a dent in a DC's performance.

As has already been commented, you don't really need to worry about adverse performance on the DC's, more so how long the query takes to run. With 14,000 objects it shouldn't be an issue, but once you start getting up into larger numbers with 100,000's of objects it may take a while to complete, and ADWS (which the PowerShell cmdlets rely on) has a default query timeout of 30mins, so if it runs longer than that it will timeout.

As already stated, you're probably not going to weigh down AD with this one query.

I do want to point out though that one query often becomes a scheduled task and code gets reused. Eventually, this one query can become thousands and thousands of generic queries will really tax AD.

The more limited to make your query the faster/better it will run. For example

Get-ADUser -Filter *

Is far slower and more taxing than

Get-ADUser -Filter * -SearchBase "CN=Users,DC=Contoso,DC=Com"

Get more specific with your queries where you can and make sure you avoid doing too much automation with them as those queries become thousands and you're in a mess.

Below is a link to page on doing efficient queries. It is written with a developer in mind so it is using VB/C++ and the like, but a lot of what it is covering translates to Powershell easily.

https://docs.microsoft.com/en-us/previous-versions/ms808539(v=msdn.10)?redirectedfrom=MSDN#creating-efficient-filters-and-other-tips

You probably don't have to worry about the AD side of things--I would test each way out and see just how long it takes to run. Go with the one that takes the least amount of time and if you can, schedule it to run at a time when a lot of users aren't going to be logging in/out (eg: not 9am or 5pm)

I'm betting this will run the fastest, even though it's querying the most accounts:

$list = 'user1','user2' # Import your list of users here
(Get-ADUser -Filter *).Where({$_.samAccountName -in $list})

You could also test out using ADSI for querying the users one by one, might be faster than Get-ADUser

$users = 'User1','User2' # Import list of users here

$Results = ForEach ($user in $users) {
    $ad = [adsisearcher]"(&(objectClass=user)(objectCategory=user)(samaccountname=$user))"
    
    # Add additional properties here
    $ad.PropertiesToLoad.AddRange(@('samAccountName','distinguishedName'))
    $ad.FindOne().ForEach({
        $obj = [PSCustomObject]@{}
        [PSCustomobject]$_.Properties.GetEnumerator() | ForEach-Object {
            Add-Member -inputObject $obj -memberType NoteProperty -name $_.Name -value $_.Value[0]
        }
        $obj
    })
}

$Results

[deleted]

I had to do something similar, I found it necessary to learn how to do hashes, I ran the powershell to grab all the users at once and all fields I needed then injected some of the fields into 4-5 hash variables, then did the lookups locally, compared to running individual user lookups it made the script run in 4 minutes instead of 4 hours. My use case was given a list of 1k random users ids, find their entire reporting chain up to the CEO and output names+chain to a csv along with email addresses. Pulling this user data of around 10k was negligible performance on the DC (multiple DCs with 4cpu and 16GB each, this probably maxed out 1 cpu on one DC for 1 minute).

Everyone is correctly telling you that you can just run the first one without worry (I routinely run it on about 20,000 users), but want to point out that you can do this one a lot more efficiently:

$results = @()
foreach ($username in $usernames) {
    $results += Get-ADUser $username
}

This will use less memory and run faster because it will only create one array instead of creating one and then copying it with the new element every loop:

$results = foreach ($username in $usernames) {
    Get-ADUser $username
}