Can you think of any change in the framework that might have caused that? Perhaps new events? Can you give me any hints of what might cause that, point me to the right direction?
It always like how a new release is generally the first casualty when something changes in an application 
Nothing has changed. We have forms far larger than this in our medical software and we have received comments from people in the field how much faster their environment has been with the new release. So this is generally a misperception.
I would place stop watches and determine where you speed issue is so that you can logically address the issue. There is nothing that we have changed that would have caused this. As for events, this would definitely not be the issue since an event it 100% lightweight if it is never handled. So if you have not handled any of the new events, then it would not add anything to your load-up time.
I recommend digging a little deeper so you can actually see where your performance issues are. From what you described, I could see some load-up issues here since you are trying to pre-load so much. So I would imagine that your issue would be in the area of retrieving data from the server...once you know where your problems are then you can start to address them.
I recommend looking at the new help documentation and the FillMultipleDataTables method on the businesslayer so that you can load all of your child records in a single trip, etc.