Hello,
Depending on your device, the download of multiple files can be quite resources hungry. To make it short:
- Every file needs to be downloaded on your web browser
- Then they are decrypted
- And finally they are converted in the format you want
If your device is a bit old, or under-powered, it can take some time. Same if you use a Virtual Private Network (VPN), or have a bad downlink connection from your Internet Service Provider (ISP).
I would recommend trying with smaller batches of files and see how it goes.