Adding the Zapier API to the app won’t make a difference. It only sends data outside of the app to trigger a zap. It does not bring data back in. It may or may not kick-start a sync process sooner depending on if sheet data is updated from the zap, or if data is updated in the app (which would trigger a sync anyway). Either way, there is still going to be a delay. Either you choose to build the logic in glide and have it calculate instantly directly on the user’s device with zero wait time, or you choose to have the logic in the Google sheet and wait for data to go from app → to glide → to google → to glide → to app.
No, I am not saying that all data needs to move to a glide table. You can move it if you want, but at the very least, leaving the data in the google sheet and moving calculations and formulas into glide computed columns will make a massive improvement to the user experience because there will be no wait time.
There is no easy way migrate data and/or formulas and there are no shortcuts. What I’ve done with my app is to move small pieces of logic into glide slowly piece by piece. It’s made huge improvements on the response of my app. You don’t have to move everything, but if there are parts of the app that are causing the most issues, then start there and see if you can make it more efficient.
Again we can only give general advice since your team has not shared much in regards to the logic that is causing problems and what exactly is slow.