No problem.
Without some way of knowing what "normal" is that additional 15-20% use of the CPU is likely to be expected.
Having two monitors and no additional processing power via a separate GPU means that the CPU has to do all of work.
Here is a link to help explain it all. You can easily find similar links.
https://techguided.com/gpu-vs-cpu-whats-the-difference/
What you may be able to do is some testing.
Start with just the laptop and its screen/monitor. Test and observe CPU usage just after boot, then while doing light work, then more online work, and finally some games. The plan in general would be to increase the graphics work load on the CPU.
Then repeat the process with the second monitor in use. Again watch the CPU.
For both sets of testing: Watch the CPU and to what extent (%) it is being used, and what is using the PSU.
You can also do the testing with Task Manager and Process Explorer (Microsoft, free).
https://learn.microsoft.com/en-us/sysinternals/downloads/process-explorer
Only use one tool at time.
Hopefully you will see a steady pattern showing that the more video being watched the more the CPU's work load increases.
You may also discover, for example, that some other application is being launched or triggered when the second monitor is being used.
Just take your time - no need to rush. Keep notes.
Post anything that is unexpected or perhaps may be contrary to the testing.
No harm in watching RAM and/or disk activity as well.