Nice to hear @benschilders and thank you for your follow up since we all can learn from that.
There's another way to go with this, since it does takes time to load in the ifc's in a project. You can't do this in batch and it needs to be done one by one so it will need your continuous attention. And you can only go on with number 2 if number 1 is finished loading. So I tried the following and this worked even quicker.
I had to convert 12 IFC's to RVT's yesterday. I opened Revit2019 12 times. Went to the first Revit version, file>open>ifc and opened ifc 1. While it was loading, I switched to Revit 2, opened ifc 2 and so on. When I was finished opening 12, 1 was already loaded. Go to 1, save as rvt, collaborate, done. Go to 2. And so on. It took me only 2 hours to convert 12 IFC's, convert them to RVT and have them collaborated. Those were giant IFC's of almost 1Gb, they ended as a 30Mb RVT. Link those with external resources in your main model and you're done even quicker.