Will it be bad to adjust the speed of the video card fan by yourself?

There must be an impact, but it is feasible for the service life of your gpu. Change the fan early at most. The design life of the video card fan is over 30,000 hours. Even with full load, the life of the fan can be reduced by 18000 hours, that is, 4 years (calculated by one day 12 hours). Changing the video card fan for 4 years is not a loss.