Hello natjes,
Thanks for using the Apple Support Communities. We understand that you're attempting to move files to your Trash however the process is moving slowly. If you look at the files that have been moved or are in process, does it appear they are moving or do things look stalled? If so, try restarting your Mac to see if that helps.
As for the speed of the process, it can vary based on the file size.
You might try booting into safe mode so that your Mac can perform a basic check of your disk, like First Aid in Disk Utility. This process will also delete some system caches, such as font and kernel caches which could be causing your system to process slower than expected.
How to use safe mode on your Mac
Best regards.