Why synchronisation takes some time?

All changes are synced via small incremental files, they are called transactions logs. Every single change generates new log. These logs should be applied in the exact same order on all your devices. The sync issues or delays happens when these logs are delivered with delays in random order to your devices.

Simple text notes will be synced faster than notes with images due to note sizes. Internet connection speeds might affect the synchronisation speed for al your notes, since all changes needs to be delivered from the iCloud servers to your devices.

Macs are downloading all changes from the iCloud as soon as they become available. Therefore macs will synchronise your changes as soon as possible.

The iOS devices are fetching iCloud changes later due to power savings and many other optimisations. It means that time to time you might get sync issues on iOS devices.

There is no direct control on iCloud from the app side, therefore all apps are waiting for changes to by synced to various devices.