Well, that's one of the main reasons why I disagree with the approach taken by many in the forum of just putting everything that affects their lives and can perhaps be better managed with other apps into Apple Notes alone.
With iOS 18 and MacOS 15 in September last year, bugs caused Apple Notes to lag massively on both the iPhone and the Mac. Apple did not fix this for months. It was only with iOS 18.4.1 and MacOS 15.4.1 that something was improved - but the problems were not fixed. Apparently you are also one of those affected.
In this Apple Support Forum thread, there are a few "workarounds" that have helped some people. However, Apple has still not solved the problem.