FCP/LCP/TTFB only show mobile thresholds

Whether to have the same or different criteria per device 

Mobile and desktop usage typically have very different characteristics as to device capabilities and network reliability. This heavily impacts the "achievability" criteria and so suggests we should consider separate thresholds for each.

However, users' expectations of a good or poor experience is not dependent on device, even if the achievability criteria is. For this reason the Core Web Vitals recommended thresholds are not segregated by device and the same threshold is used for both. This also has the added benefit of making the thresholds simpler to understand.

Additionally, devices don't always fit nicely into one category. Should this be based on device form factor, processing power, or network conditions? Having the same thresholds has the side benefit of avoiding that complexity.

The more constrained nature of mobile devices means that most of the thresholds are therefore set based on mobile achievability. They more likely represent mobile thresholds—rather than a true joint threshold across all device types. However, given that mobile is often the majority of traffic for most sites, this is less of a concern.

The Web Vitals initiative recommends the same thresholds regardless of device for its thresholds, which basically uses the mobile thresholds. However, since Lighthouse is a lab tool, it can be useful to be stricter when testing for faster desktop devices it uses different thresholds.