How can I reduce my MacBook Pro 2015's CPU clock speed or power consumption? I want the battery to last a lot longer.
This seems to be a duplicate of your other question.
Apple designs its laptop for the best balance between performance and battery life. There is very little you can do to make the battery life 'a lot longer' -- while still using it to do useful work.
Lowering the clock speed, if possible, would reduce what you are usefully able to do with your machine.
Apple claims 'up to' 9 or 10 hours 'wireless web browsing' or movie playback on 2015 models -- with the emphasis on 'up to'. You're not going to see more than that, whatever you do, and if your battery is 7 years old, you'll be lucky to get half that.