You are certainly planning head. but you are absolutely correct, that like a frog getting slowly boiled, apps do slow down as the amount of data increases.
If you have the luxury of lots of room in your plan for the # of apps, the easiest way is to simply make copy of your app and then lock down the role access to view only. Before you do that you would make some reports in your large tables to purge out old data (but do not yet purge).
Then you would copy the app with data. Then you can go ahead and manually run the purge report and use the More .. button to delete the old records.
You may choose to purge the new data from your archive app, just so there is no overlap.