I believe it’s the whole spreadsheet. I think of it this way…sometime Glide needs to transfer all of that sheet data from the google sheet to Glide’s internal copy of the data…especially when doing a reload. I think normal data syncing is in small chunks, but a reload probably pulls the entire spreadsheet. Also consider how many columns are involved. More columns means more data in each row to move as well. I really don’t know the inner workings of all of this, but I imagine Glide starts to run into timeouts when trying to pull and process large datasets. When Glide Sheets are released, this will give us the option of using sheets that are purely contained within glide, so this would help to eliminate this technical bottleneck between Glide and Google. In all honesty, I have done a test with up to 120,000 rows but maybe 3 columns. It worked for the most part but really slowed down my computer and phone. Mostly because I probably didn’t have any filtering and was trying to load all of that data at once (not a typical use case).
So 25,000 isn’t a hard limit, but a general guideline due to technical limitations. More is possible, but if it’s complex data, then you may have problems. 25,000 in a single sheet may or may not work, but you need to also consider the amount of data that will be syncing and caching to each device as well.