In 1.14.4 Mojang introduced debug reports to help with finding and fixing performance problems and bugs, e.g. low mob spawning.
To create such a report, you have to execute the command
either in the console or in-game (with OP permissions). Your server has to be online to execute that command.
After that, you can find and download the report (a zip file) here: https://aternos.org/files/debug
Please note, that the download is only possible while your server is offline.
You can find the Mojang bug tracker to add your report to an existing issue or to create a new issue here: https://bugs.mojang.com/projects/MC/issues