You were told to cast the variables as CDBL() not the result!
The code below works for me.
Code:

$strComputer = "."
$objWMIService = GetObject("winmgmts:\\" + $strComputer + "\root\cimv2")
$colItems = $objWMIService.ExecQuery("Select * From Win32_PerfRawData_PerfOS_Processor Where Name = '0'")
For Each $objItem in $colItems
  $CounterValue1 = cdbl($objItem.PercentUserTime)
  $TimeValue1 = cdbl($objItem.TimeStamp_Sys100NS)
Next
For $i = 1 to 5
  Sleep 1
  $colItems = $objWMIService.ExecQuery("Select * From Win32_PerfRawData_PerfOS_Processor Where Name = '0'")
  For Each $objItem in $colItems
    $CounterValue2 = cdbl($objItem.PercentUserTime)
    $TimeValue2 = cdbl($objItem.TimeStamp_Sys100NS)
    If $TimeValue2 = $TimeValue1 
      ? "Percent User Time = 0%"
    Else
      $PercentProcessorTime = 100 * ($CounterValue2 - $CounterValue1) / ($TimeValue2 - $TimeValue1)
      ? "Percent User Time = " + $PercentProcessorTime + "%"
    Endif
    $CounterValue1 = $CounterValue2
    $TimeValue1 = $TimeValue2
  Next
Next
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