While some complaints have been seen here - this being the case for any software update ever released - the vast majority of end-users have no significant issues with iPadOS 26. At the time of writing, iPadOS 26.1 is current.
That said, iPadOS 26.x has significant differences that change both the appearance (Liquid Glass) and multitasking capabilities and features - both of which will doubtless take you a tittle time to become fully accustomed. Some of your workflows will doubtless change as you learn the updated gestures and controls.
As with all system software updates, post-update you should anticipate seeing short-term impact upon system performance - as your iPad will (a) be performing background housekeeping tasks for several days, and (b) you will be exploring the new UI and features. Normal performance can be expected to return within a few days of update.
At some point in the near future, security updates for iPad models running iPadOS 18.x, that are capable of update to iPadOS 26, will cease to be available. At this time, the option to safely continue with iPadOS 18.x will be lost - necessitating update to the most recent supported version. As such, to maintain protection from emerging security threats and vulnerabilities, remaining with iPadOS 18.x is a short term option.