The “Page Unresponsive” (wait) warning is triggered by your browser (especially Google Chrome) when a web page takes too long to respond, usually due to heavy processing. In the case of the Mass File Download app, this occurs during large file downloads when the browser is zipping and preparing many files at once using JavaScript.
This behavior is controlled by Chrome as a safety feature to prevent tabs from freezing, and unfortunately, there’s no browser setting to disable or delay it. However, the process will still complete successfully. If you see the warning, simply click “Wait”, and the download will proceed.