If you do not need to show the field to certain users, it might be best to simply set up permission for each field at App setting from Kintone basic feature.
▼ Setting Field Permissions
However, if you need to customize with JS, you might want to reconsider what is causing the delay in your current coding. For example, whether the executed API process was working with an immediate function or not.
I also think in case of heavy processing or pre-processing of the synchronization process, there might be a slight delay in processing.
As another possibility to solve your issue, you might want to use a loading icon, spinner, with the spin.js library. While the spinner is working, you may be able to show some kind of a dialog that covers the entire window to hide the page until the API processing is complete.
This website below explains how to utilize spin.js.(sorry but without a dialog to cover the entire window as I suggested)
▼ Show/hide a spinner with spin.js
Hope it helps.