@ Peter ... sorry you have not found a solution. "Loading (very slow, with a lot of "page doesn't react"-warnings) " indicates the issue you are having is most probably in your Project code or issues with your Internet connection speed.
Your project is probably very large, has lots of blocks, perhaps lots of screens and layout; perhaps large image or sound files. You have not posted an aia or images of your blocks so no one can say for sure why you are having so much trouble while the majority of users are not. Most probably, "I think waiting untill a solution with a new version is my only option." is not going to work. A new version of AI2 will probably not cure your difficulties. MIT will not return to version 144 of App Inventor.
Here is a huge list of things a developer might check to improve performance I imagine one or more of the following issues possibly applies to your code (all may not apply to your code):
You might be able to use one of the third-party versions of App Inventor that MIT does not support to ameliorate your issues. This approach might work for you if you were happy with the older version of MIT App Inventor. You might query Hossein with respect to his third party AliveComplete. There is a free version...you have to run the server on your computer..what you would want is to find out if he has a version of AliveComplete that runs on version 1.44. If he has one, get it and use it to develop.
Using this solution: you probably would never be able to use MIT App Inventor on the Cloud again with your existing code;
you would never get updates that make AI2 more exciting for other devices but have to rely on the third party to incorporate those features
you could probably update your existing apps using the tools available to that AI2 version (see below) and they should run on at least some of your devices. I do not know, you would have to try.
Continuing to use MIT AI2 would be like like riding down a country road to the freeway (live with the bumps to get to the pavement), using an old version of AliveComplete would be like riding down a country road that ends in a swamp but stop short of the bog and you might be happy.
To get out of your mire, you could try an older version of AliveComplete (learn to use it) and might get some JOY. You could post an aia of an example your Project and someone might look at it. You can also start the painful process of modifying all your app and layouts as might be necessary to work with AI2 version 1.46+