CPU frequency drops and Low performance

Minecraft 1.14.2 seems to drop CPU frequency from 3.1GHz to about 1.2GHz, which brings really poor performance (10-30 FPS) and also slow down other programs massively. I noticed this problem from MC 1.13 but it became so significant in MC 1.14.2 that I have to stop playing and report it. CPU temperature is good and not able to cause throttling. Sorry for my poor English.

 

(Note: In the attached images, the left parts show the situation with Minecraft running, while the right parts show that without MC)