ValueFromPipeline processing in a function is serial unless you use a steppable pipeline

parallel-begin-process-end.ps1

#Requires -PSEdition Core

# HT @santisq in powershell-slack
# https://powershell.slack.com/archives/C1RCWRDL4/p1685120926413089

function foo {
    param(
        [Parameter(Mandatory,ValueFromPipeline)]
        [string]
        $bar
    )

    begin {
        $ScriptBlock = {"input is $_"; Start-Sleep 5}
        $pipe = { ForEach-Object -Parallel $ScriptBlock }.GetSteppablePipeline()
        $pipe.Begin($PSCmdlet)
    }
    process {
        $pipe.Process($bar)
    }
    end {
        $pipe.End()
    }
}

$foo = @('a','b','c') 
Measure-Command {$foo | foo} | select TotalSeconds
Measure-Command {foo $foo} | select TotalSeconds