The X1 Yoga performs well under stress but experiences small spikes of high temperature. Running Prime95 to fully load all cores, the Core i5-8250U is able to boost to its rated maximum of 3.4 GHz, but temperatures rise to 97-98° C shortly after hitting this peak. This chokes the CPU to about 2.4 GHz in order to bring temperatures down (about 85° C). Running Prime95 and FurMark concurrently forces the CPU to pull back even further to 1.4 GHz, 200 MHz below its base clock. Again, there are intermittent boosts to 3.4 GHz, but these are short lived. The system doesn't seem to manage synthetic graphical stress well; FurMark (by itself) is able to run for about 20-30 minutes before crashing. CPU temperatures peak around 98-99° C prior to a crash, suggesting that heat limits are responsible for the failure.
Unigine Valley gives us a better idea of a more realistic load. Under this test, the CPU typically hangs near its base clock of 1.6 GHz, boosting as needed. The GPU performs well, clocking in at about 1100 MHz. Temperatures across the CPU package stay well within check, average about 79° C. Temperatures of 90° C can be recorded, but only after a single core hits its maximum rated boost. Overall, temperatures and performance are well-balanced under real-world stress.
Source : https://www.notebookcheck.net/Lenovo-ThinkPad-X1-Yoga-2018-Core-i5-8250U-FHD-Convertible-Review.299148.0.html