Parallel cout is very sluggish when displaying the progress percentage

After git pulling the newest Darwin&Core repositories, using parallel to run command is very slow when cout the progress percentage.

The process runs smoothly at the beginning, but becomes very sluggish in the latter 50%. The completion time of the process cannot be output immediately; sometimes it takes several minutes to display the previously completed information. When checking the process in htop, it quickly enters the sleep state. It works fine without using parallel.