After testing a few fast browser games on mobile, I found that simple running games are good for checking touch response because small delays are easy to notice. Slope is one of the better examples because the controls are simple but the speed increases quickly.

For comparison, I used this version: https://slopeunblocked76.net

On Cubot phones, the game felt smoother when browser zoom stayed at the default level, heavy background apps were closed, and battery saver was turned off during short play sessions. Landscape mode also helps because there is more room for left and right movement.