Hi Thijs,
I think you should have a closer look at the DropZone Widget. Is the the basic upload of a single file using the standard widget (not the dropzone) also slow? Did you have a look at the browser console if there is a data traffic issue ?
I believe the way that the Mendix UI communicates with the server runtime changed in this span between 6.6 and 6.10, specifically in 6.8 (see these release notes). Specifically, I believe it sends changes made to objects in the browser alongside microflow calls or other server requests instead of sending them in a separate stream.
I wonder if this is an edge case where that actually decreases performance. Could you share a HAR file from the browser for your app in 6.6 and 6.10 so we can analyze the difference? This is done in the "network" tab of your browser's developer tools. Here's an article from Atlassian that overviews the general process of exporting a HAR file.
This should help us determine whether the problem is due to a change in network communication or a change within the runtime.
EDIT: By the way, if you have the opportunity to create a small test project, it would be great to submit that along with a support ticket.