Intel CPUs has something called turbo boost that will increase the speed of the CPU if there is a demand for it and if the CPU's temperature is below a certain level (something that Intel sets). While the i5-8250u CPU can achieve up to a max speed of 3.4GHz (when only one core is used), that is only for a short period of time.
Reviews have stated that under a constant heavy load the sustained max speed is around 2.4GHz. The i5-8250u is a "u" model CPU. The "u" stands for ultra low voltage which means laptops with "u" model CPUs have small heatsinks since the CPUs are not designed to generate a lot of heat. One way to prevent the CPU from generating too much heat is by limiting the clockspeed.
If you want a laptop that can sustain higher max clockspeeds, then you need to look at "HQ" model CPUs which will have larger and heavier heatsinks to dissipate the heat.