Thank you for the answers and sorry for my late response.
I created a smaller project to present my problem, because it is not related to the API, stored variable or the number of the rows like @tatiang suggested.
I have 4 examples on how the Data Viewer List/Grid gets slower and slower. I hope that it can help the team to debug the issues in Thunkable and fix this.
Project link: Thunkable
I just create a table without any Data Viewer on the screen. It took: 0.16 seconds for me. I think that should be the optimal.
Creating the same table, but in this case I have a Data Viewer Grid on my screen without any Data Bidings. It took 1.26 seconds for me. I mean I could live with that, but it even get worse in the next example and it is a more than 7 times slower than in example 1.
In example 3, I had the same table, the same DVG, but now I had Data Bidings. It took 2.96 seconds to load, which is more than 18 times slower than example 1.
In this example I changed the code a little bit, I added a simple if block inside the loop, and I got 4.82, which is extremely slow and is 30 TIMES SLOWER than in example 1.
I understand, that it should load slower, because displaying the items in the Grid, but this is too slow, waiting 4-5 seconds to display a calendar like I want to is extremely slow.
I don’t want to clone blocks as @jared suggested, because my Data Viewer Layout contains more components and detecting a click event for all of them would be impossible. Or do you have an idea how the click event for a column like this should like?
Thank you again for your time and answers and I hope that @Thunkable_Staff will have a look on this.