Beta 1 versions are usually quite laggy, especially on devices with older hardware like the iPhone 11 Pro Max. That’s completely normal, and Apple will gradually improve performance in later beta releases.
I usually restart my iPhone if it’s not behaving as expected. However, if you feel it’s affecting your daily usage, I recommend downgrading to iOS 18.5 or waiting for more stable iOS 26 beta updates.
You can find the guide for downgrading without data loss here: