The "U" series are low power series, made for laptops. It's a great CPU, but it's meant to consume little energy. If you want it to maintain for long times a full load, it will throttle (lower its speed) to keep power consumption low. The more powerful a CPU is, the more heat it generates, and in a laptop you can't dissipate lots of heat easily, hence the low power CPUs. There are ways to use beefier CPUs in laptop, but it costs far more moneys 😉