? The base clock speed doesn't change. It's 100MHz all the time. The clock speed of the CPU won't go lower than 1.6GHz and might hit as high as 3.4GHz depending on temps and usage. If you don't know how turbo boost works, I suggest reading this (and other material) and then asking what you don't understand.
Yes, the base clock speed can have an affect on performance. Obviously the higher the base speed the better the performance will be.
Intel uses Turbo Boost and Windows uses power management settings to determine the clock speed. Based on how much the CPU is being stressed. For example, I am currently replying using one of my older laptop; a Dell Latitude 3540 with a dual core i5-4200u (1.6GHz / 2.6GHz); from 2013. Since all I am doing is typing the clock speed basically ranges from 800MHz but it does go up to 1.6GHz sometimes and can also go up to 2.3GHz from time to time. It hits 1.6GHz and 2.3GHz because Windows does have some background programs running that puts a demand on the CPU. Note that 2.6GHz is only possible if only one core is being used; if both cores are used then the i5-4200u will only go up to 2.3GHz.
When it comes to the quad core i5-8250u; 3.4GHz is likely only achieved when at most 2 cores are used, more than that means the max speed will like drop to 3.2GHz or 3.1GHz. Having said that though the i5-8250u is only a 15w CPU so most of the time it will stay below the max speed to control power consumption and heat. Based on some reviews, when the i5-8250u is being stressed for a prolonged period of time the CPU hovers around 2.5GHz or so; that was during video encoding which used all 4 cores. While the CPU can achieve 3.4GHz / 3.1GHz it will only be for a very short period of time and only if the CPU temperature is not too high.