Craft 3 CP performance
Is this a problem with only the dashboard, or the front-end also? Although this is a bad answer, waiting a few seconds to load the dashboard shouldn't be a huge deal. You will only need to access it a few times while editing content. On the other hand, there are ways to improve the front-end performance such as Eager Loading. 

You can approach this conceptually by creating a simpler data model. In my opinion, super table inside of a matrix is a sign that you are doing something wrong. There is a reason that matrix in matrix is not supported, probably because of this performance issues like this.

You can also extend the dashboard or something to implement eager loading or cut down on db queries but I don't think it is worth the effort. Maybe you can mention something to Pixel and Tonic and they will improve this in a later version of craft.