Sorry for the late reply, but it took longer than expected to get all the modules ported to 9.4.1.
After the upgrade we now get a double B rating on gtmetrix, instead of double A, which is better than I expected. Before we had moved all CSS and JS to the CDN and now that stuff is on the local server again, for example. I hope to get to optimizing it during this week. Am happy that it's all working for now. :)
Did you try out the Client Resource Management? You can find it in Persona Bar -> Settings -> Servers -> Server Settings -> Performance -> Enable Composite Files. I haven't tried it yet but it is supposed to combine CS and JS files to reduce the number of requests. I see that we have 25 requests now oposed to 14 before the upgrade. Enabling this option should reduce it hopefully. There is also options to automatically minify CS and JS files, a task that I had to perform manually with the old DNN version.