Create a mod with a lot of dependencies. It does not matter which, just make sure it's a lot (>50). This also applies to indirect dependencies.
Try to upload the mod. When you first click the button in the toolbar, you will have to wait 1-2 minutes (100 dependencies) while it's loading (Publish Project window is greyed out). Then you get the ability to enter the form and click publish. At that point it will grey out again, and you need to wait another 4-5 minutes. Only after that does the small "Publish Addon" window appear where you click yes. From then on, everything is normal, packaging and uploading does not take any longer than normal.
A mod with no / very few dependencies will obviously not take so long for that. Maybe there is something implemented badly on the Workshop backend, I don't know.
If you ask yourself why there is a need to upload a mod with 50 (or in my case, 100+) dependencies: E. g. server specific mission packs with lots of content that can be used for the missions.