As the App gets bigger, firstly you see sync times increase. Then you’ll get reports of syncs failing on mobile deivces. Because their data is slower and it times out before it completes. Then you gets a few times where doing a Save & Verify fails because there are so many things for it to check. Then if you really try hard you can even get it to fail to save. I only know because I’ve been through all those stages 
To speed up an App, in order of action, my quick 20sec summary would be:
- Make sure all performance settings are turned on
- Use the bar graph in Performance Analyzer to find the slowest part of the app and try to make it more efficent or get rid of it
- Remove all virtual columns. If needed have it run an action when you save a form that mirrors what the virtual column did
- Sometimes slices seem to be more efficent at generating tables of data than virtual columns
- Move any calculations into Bot template files if they aren’t needed in the app
- Use security filters to limit the data if your on a suitable licence. Otherwise create a bot that copies data to another spreadsheet
- Delete rows and columns you don’t need
Blank rows do take time for the app to read. But I think it probably adds a couple hundreds of secs to the sync time. Having said that I always delete any extra columns and the same with rows where i know the App won’t be adding any more.
Hope this helps 