Build for the phone your users actually own
To build software for low-end phones, keep pages light, put the most important action first, support offline use where it matters, and test on the slowest device your audience is likely to use.
The gap between the office and the user
Most software is built on fast laptops and tested on new phones over good connections. Many of the people using it have none of those things. A page that feels instant in the office can take a long time to load on an older phone with a weak signal, and many people will leave before it does.
Decisions that change
Designing for a low-end phone first changes the decisions a team makes. Pages get lighter and images get smaller. The most important action comes first, because a small screen leaves no room for anything else.
Saving work offline stops being an extra and becomes part of the plan. If someone fills in a form on a bus and the signal drops, their work should still be there when it returns.
A short checklist
Test on a real, inexpensive phone, not only a simulator. Slow down the connection during testing. Set a limit on page size and check it with each release. Keep buttons large enough for a thumb.
These choices help everyone. A product that runs well on a cheap phone with a weak signal will feel quick on an expensive one too.