"Backwards Read Primes" of Codewars powershell version
Update: Later this night after I asked my question I continued to think of ways I could have improved the performance of my solution. I made some refactors that I'm going to share which gave me enough boost to beat the time requirement for this problem.
But please continue to post your answers, I'm here to learn as well. And any new nugget of efficiency I can add or look at is still super valuable to me.
Thanks in advance...
So the fist changes I made even though a bit small was
I removed the .ToString() to my $num in my isPrime function, I realized that it was not even needed, and yes that's a small change but every bit helps in this case...
The update isPrime function below...

  function isPrime ($num) {

      if ($primes.containsKey($num)) {
          return $true;
      }

      if ($num -lt 3) {
          return $num -gt 1;
      }

      if ($num % 2 -eq 0 -or $num % 3 -eq 0) {
          return $false;
      }

      $i = 5;

      while ($i * $i -le $num) {
          if ($num % $i -eq 0 -or $num % ($i + 2) -eq 0) {
              return $false; 
          }

          $i += 6;
      }

      $primes[$num] = $num; # removed the .ToString() here... very small optimization but it all matters I guess.

      return $true;
  }


Next I went to the for-loop and realized that I was doing 2 isPrime calls and a reverse number call as well... It dawned on me that I could easily just declare and assign $nPrime variable and then check if it's false then no need to continue and just go to the next iteration, same with $nPrimeReverse. But before that I would call revNum and test if $nReversed is equal to $i if it is, then also I would call continue and start the next iteration. Then finally I just add a check at the start determining if $i is greater than 13.
Here is the updated for-loop:

  for ($i = $start; $i -le $stop; $i += 2) {
  
      if ($i -lt 13) { continue; }
  
      $nPrime = isPrime $i;
      
      if ($nPrime -eq $false) { continue; }
      
      $nReversed = revNum $i;
      
      if ($nReversed -eq $i) { continue; }
      
      $nPrimeReverse = isPrime $nReversed;
      
      if ($nPrimeReverse -eq $false) { continue; }

      $res += $i;
  }


After all these small changes my solution passed within the time constraint.
Please feel free to comment any additional items that I could refactor and improve.
Warm Regards