No, Internet Download Manager is not dangerous for the performance of your system. Its memory footprint is rather low, and it doesn't interfere with the performance of the PC. I have been using this application for many years and never had any issues regarding performance. Of course, bugs in the application might appear from time to time, but that doesn't have anything related to system performance.
However, I recommend that you always install the latest version of the application, no matter what.
Download page: https://www.internetdownloadmanager.com/download.html