How to compare processors with "Turbo Boost"?

The answer is complicated.

Let's assume you don't play with your bios settings. Depending on the load of your computer and the heat it produces it will spin a single processor core up to 3.0 GHz, it may spin a pair of cores up to 2.5GHz and all 4 cores up to 2.1GHz as long as the processor is not overheating but will only guarantee 1.8GHz.

This is only an example from memory and may not be correct for this particular processor but is a rough outline of what it does.

The purpose of Turbo Boost is to provide additional performance when the CPU is being stressed. Typically the clockspeed will bounce between 1.8GHz and whatever the lowest clockspeed is. Let's say it is 800MHz. I think if the CPU is being used to the point where it is operating at 1.8GHz for a short period of time (say 1 or 2 seconds just for explanation purposes), it will then boost the clockspeed up to a max of 3.0GHz.

The Core i7-4500U is only a dual core CPU. Only the Core i7 "MQ", "HQ", and "MX" CPUs are quad core CPUs. If only one core is being used, then the CPU will max out at 3.0GHz. If both cores are used, then I think the max clockspeed is 2.8GHz.

In general, the extra performance is there if it is necessary. However, the higher clockspeed also increases power consumption and creates extra heat. If the CPU gets too hot Turbo Boost will be deactivated and the CPU will throttle down below 1.8GHz, maybe as low as 800MHz to prevent heat damage.

"U" CPUs are ULV (ultra low voltage) CPUs which means the CPU speed without Turbo Boost is limited to below 2.0GHz so that the CPU uses less power and keeps the drain on the battery relatively low.