5G speed is capped at 100 or 250Mbps, depending upon the plan. This speed cap on a 5G plan applies whether you are connected to 5G or 4G. 

This is the maximum potential download speed. Typical speeds will often be slower and will vary due to factors such as location, device capabilities, distance from the base station, local conditions, concurrent users, hardware and software configuration and download/upload destination.

Beyond the maximum potential speed for your selected plan, a speed cap does not indicate any relative performance difference compared to another plan with a different speed cap.