The information that you are giving us is the link to the folder - not the link to the individual files within that folder.
That is why "anyone with the link" can get to the folder, but cannot get to items witin it, ie because they do not have the links to the items in the folder.
Arguably this is a silly distinction: if you want "anyone with the link" to see the folder, then surely you're implying that you've given them the file links, too. But this isn't how Drive is working at the moment, according to testing I just did in Chrome.
Right now, if you want me to see the contents of files in your Google Drive folders, then you have to give me the link to theose files, not just to the folders.
(This may be different if you set permissions to Public on the Web - if you want to test this, set up a folder and I'll take a look.)
In an arrangement where each of N user's changes propagate to all the other users' screens, that incurs N2 propagation costs (network messages, recalculations, and display updates). That grows rapidly as N grows. So if N > 3 you want to avoid it for sure.
A separate sheet per user might avoid most of the updates but the best bet is to use a separate spreadsheet document for each user.
Given that, using a separate Google Account per user might improve performance a bit by avoiding synchronized updates to some shared state (e.g. the user's document list). Also this is the normal case, unlike one account editing docs from many tablets at once, and any optimizations will favor the normal case. You'd have to measure it to be sure, but my semi-informed bet is that you won't notice the performance difference.
Giving each user a separate login account would help with tracking changes, but that might not be worth your setup work.
Idea: If all the columns are uniform within each sheet, that is, if the data is like a simple sequence of records, look into using Google Fusion Tables in place of spreadsheets. Fusion Tables scale up to very large data sets since the rows are independent of each other. You can "publish" columns from one table data to other tables.
Idea: If this is not a temporary application, consider replacing the spreadsheets with a custom implementation as a web app or native Android apps. Even then, it's good to prototype your application with spreadsheets as a way to discover what really matters to your use.
Best Answer
It's not possible but it the document is shared with anyone with the link you could appear as an "anonymous animal". One way to achieve this is by open the link without being signed in in your account. On Chrome, right click the document link and open it on incognito mode.
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