First run of script is slower - powershell

Question

I am doing some performance profiling and noticed something very odd. I wanted to compare two ways of building a list of XML files. One uses Get-ChildItem twice, with the first one filtering for a single file and the second filtering based on file name, and the second approach uses Get-Item for the single file and Get-ChildItem again for the multiple files. I wrapped both in Measure-Command.
When I run it, the first run shows a much longer time for the very first Measure-Command, but it doesn't matter which approach is first. And it's only the first one over a fair amount of time. So, if we call the two approaches GIGC (Get-Item & Get-ChildItem) and GCGC (Get-ChildItem & Get-ChildItem), if I have the order GIGC then GCGC, and I run it I will see 2.5 seconds for GIGC and 1.5 seconds for GCGC. If I immediately rerun it they will both be around 1.5 seconds. It will stay around 1.5 seconds as I rerun over and over. let the console sit for a few minutes and GIGC will again be around 2.5 seconds. BUT, if I reverse it, GCGC first and GIGC second, the numbers remain the same. That first Measure-Command, this time of GCGC, will be 2.5 seconds, and all the rest will be 1.5. Let the console sit for long enough and the first one will go back up.
$firmAssets = '\\Mac\Support\Px Tools\Dev 4.0'
Measure-Command {
[Collections.ArrayList]$sourceDefinitions = #(Get-Item "$firmAssets\Definitions.xml") + #(Get-ChildItem $firmAssets -Filter:Definitions_*.xml -Recurse)
}
Measure-Command {
[Collections.ArrayList]$sourceDefinitions = #(Get-ChildItem $firmAssets -Filter:Definitions.xml) + #(Get-ChildItem $firmAssets -Filter:Definitions_*.xml -Recurse)
}
At first I thought the issue might be with Measure-Command, so I changed the code to use Get-Date as the timer. Same results.
$startTime = Get-Date
[Collections.ArrayList]$sourceDefinitions = #(Get-Item "$firmAssets\Definitions.xml") + #(Get-ChildItem $firmAssets -Filter:Definitions_*.xml -Recurse)
Write-Host "$(New-Timespan –Start:$startTime –End:(Get-Date))"
$startTime = Get-Date
[Collections.ArrayList]$sourceDefinitions = #(Get-ChildItem $firmAssets -Filter:Definitions.xml) + #(Get-ChildItem $firmAssets -Filter:Definitions_*.xml -Recurse)
Write-Host "$(New-Timespan –Start:$startTime –End:(Get-Date))"
So, last test, I thought it might be something related to the console, so I converted to a script. Weirdly, same result! First run is always substantially slower. My (uneducated) guess is that there is some memory initialization thing happening, that happens on the first run and then stays initialized over multiple uses of PowerShell, be it scripts or console. Basically some sort of PowerShell overhead, which feels like something we can't work around. But hopefully someone has a better answer than my "That's just how PowerShell works, get over it"