The ThinkPad P1 features a very powerful CPU inside a relatively small chassis. As a result the CPU fans can activate more often than is wanted and the amount of time you can spend on battery can be reduced. There are a couple of tools to help with this.
In Windows, the Intel XTU utility can be used to conserve the electric voltage that is supplied to the CPU. After downloading you can test a reduced voltage and ensure your machine is stable. The following guide has further details on how to use this utility:
If you don’t want to alter the CPU settings than another alternative is to alter the Windows power settings. From https://www.reddit.com/r/thinkpad/comments/9nu30b/anyone_have_a_new_p1_thinkpad_with_xeon_and_ssd/:
Go to settings > power and sleep > additional power settings
Change the power plan to “power saver” and then click on “change power settings” and then click on “change advanced power setting”
Then scroll down to processor management and select maximum processor state and set “on battery” to 99% and “plugged in” to 99%.
Then go back to processor management and select system cooling and set it on “passive” for both “on battery” and plugged in”