Despite the fact that the 8th Gen. Core i7-8xxxU processors are marked in ARC as being Mobile processors, the "U" is an indication that the processor is in the Embedded family. This means that they are SOC packages with chipset (PCH component equivalent) included in the package. They are also only 15W parts - and, despite the improved performance the 8th Gen. parts are supposed to have, you know that they are not going to have the same level of performance as the 45W (or higher) parts that are available (even those in the 7th Gen.). To run in this power envelope, some things had to give - lower clock rates, lower turbo boost settings, etc.
That's the high level view...