The good news
The KhoindVN APK supports Android 5.0 (Lollipop) and newer, which means it officially runs on phones pushing a decade old. That is not an accident: a WebView wrapper has almost no native code to age badly, so old phones are first-class citizens here, not an afterthought.
The one thing to update: WebView
On old phones, the critical component is not the app but Android System WebView, the system engine the app uses to display the site. If WebView is years out of date, pages may render oddly or not at all. Open the Play Store, search for Android System WebView, and update it. On Android 7 and newer, Chrome updates can serve the same role.
This single update fixes the large majority of "blank page" reports on older devices. It is worth doing before anything else.
Performance expectations
Set expectations honestly: the app itself will open instantly on any hardware, because 13 KB opens instantly everywhere. Page loads then depend on the site and your connection, exactly as in a browser. An old phone with 2 GB of RAM will feel the weight of a heavy web page, not the weight of the app.
Close background apps before a reading session if your phone is very old; WebView shares RAM with everything else running.
Install notes for old Android
The install flow on Android 5 through 7 differs slightly from modern versions: the unknown-sources toggle lives in Settings, Security, as a single global switch rather than a per-app permission. Our install guide covers the modern flow; on old Android, flip that one switch, install, and flip it back.
If the installer refuses with a parse error, re-download the file and confirm it is about 13 KB; interrupted downloads are the usual cause, and the fix guide has the full checklist.
Is it worth it on a 2016 phone?
Honestly, yes, and for a specific reason: old phones benefit most from small software. A 13 KB app that asks for one permission is kinder to aging hardware than any browser with twelve tabs. Readers on old phones are exactly who lightweight apps are for, a point we make at length in the lightweight case.