Mobile scores are often lower because the test represents a more constrained device and network. It is not simply a smaller version of the desktop test.
Mobile has less room for waste
Large JavaScript bundles, oversized images and third-party scripts compete for slower CPU and network resources. Work that feels instant on a powerful laptop can block a phone for much longer.
Responsive does not automatically mean fast
A layout can fit a phone while still downloading desktop-sized images or executing the same heavy scripts. Responsive design solves presentation; performance work solves delivery and processing.
What to fix first
- Start with the largest visible content and its source image.
- Reduce JavaScript needed before interaction.
- Delay non-essential third-party services.
- Improve server response and caching.
- Retest mobile and desktop separately.