We have a grid with around 30000 rows with store buffered.
Every page it takes like 2-3 sec to loads and 1-2 to render (we have a lot of columns around 100 with special render functions)
I cannot reproduce on our example because of the fast loading of every page, but I think you have a problem in the bufferrender plugin.
Steps to reproduce (on a big grid with a lot of columns):
1. load the first page.
2. Drag the scroll on the middle of the view ( don't let the mouse )
3. let the grid begin the load the data for that page
4. after the grid begin load the data, scroll on another page.
5. after the grid begin load the data, scroll on another page.
6. after the grid begin load the data, scroll on another page.
7. after the first page is loading the grid now has no mask, but the grid is still loading
On 4.1.3 you have a component named PagingScroller, that in onViewRefresh function "took" care of this problem ( moving the scroll on the last page that user loads even if there are 100 pages that are loading because user did not let go of the scroollbar on every of this pages). On 4.2.1 that should be take care by the same function in that new plugin that you introduce ( BufferedRender ).
In my case that will introduce a lot of problems (a grid that loads data but have no mask) ( example ADD row feature )
Sory that I can't give you an example (with code).... but if you can make your example to load slow a page ( like a normal application ).... I can show you that problem.