Since updating to iOS 18.6 beta on my iPhone 13 ProMax, my battery usage has well over doubled. Also, my phone has become noticeably sluggish and unresponsive.
For instance, after being charged to 100% my phone has dropped to 26% with only 1 hr 2m screen on time and 49m of screen off time. My phone gets noticeably warm to the touch, even through the case.
Any ideas on the cause or any remedy?
Yes, the cause is it's a beta. Beta software contains various applications and processes that aren't present in the release versions of the software.
There are diagnostic tools and processes running in the background that allow Apple to confirm the new software is functioning correctly.
Also, on initial install of a new OS it has to perform some initial setup, including indexing.
Those processes take battery power.
You shouldn't install beta software on your primary device - for exactly these reasons.
Beta software is only really of use to developers and those needing the newest version to test something, or to develop something. If that's not you, you probably shouldn't install the betas on your devices.
If, after a few days, your device is still sluggish and the battery level is unacceptable, there may be a bug, and you can raise it here: https://feedbackassistant.apple.com/ Your alternative is to downgrade back to iOS 18.5, and restore from a backup.