The output to the internal console is slow because it relies on a timer to poll the process. I believe it will only check for pending output every 100ms. There are other restrictions on the amount of output the internal console can handle as well. I normally suggest that, for substantial output, the external console should always be used.
The external console is actually just a separate Windows Command Prompt process that Simply Fortran launches. It does not experience any of the issues related to process piping that the internal console does.
If we were to revisit the internal console to attempt to find more points where it can be optimized, it wouldn't be until version 2.29. Again, there are significant restrictions on its performance that are simply inherent in its design.