odysei, the high gpu load when using forcecompositionpipeline is a known issue, the composition pipeline has been changed starting with driver 390. To return to the pre-390 behaviour, you can use
Option "UseNvKmsCompositionPipeline" "false"
in your xorg.conf.
But this has nothing to do with the issue reported by user_0462, in that case there’s no gpu usage but a higher power consumption and no composition pipeline involved.