@gavilanpollero
Is it just one App? If you fire up another app or create a new one, do you experience the same issue?
If it is just one app, keep in mind that the designer also is live - meaning that it constantly updates things within your app as you work on it. So, if you have some formula or other action that is taking a long time to execute, that will have to do its thing before you get control back.
I find in some more process intensive formulas, it becomes very frustrating to try and make changes all the while it is trying to fetch and display the data from the formula.
In those cases, I will put some "garbage" letters at the top of the formula. This will cause the designer to see things as an error and then will not process them. You can then edit the formula easily, but you do lose some autosense capabilities. But far less frustrating.
I hope this is helpful for you.