I'm working on a site that was created a few years ago and already has GA. It also has GTM for some other tags. I want to understand is there a good reason to update the already working code and move GA inside GTM? Also, can this somehow improve or break site performance?
P. M. I understand that GTM is more flexible and you can change your tags without touching the main code. But in my case, GA is already implemented outside GTM and I need a good reason to do this change))
Thank you!
You have already ruled out the main reason (flexibility), which sounds reasonable if your requirements do not change a lot. Having everything in one place is somewhat nicer and more organized, but I assume you want to know if it is worth spending money (time) on it.
I don't really have hard numbers, but from experience (as a consultant, and now working for a large-ish e-commerce operation) I would say performance differences will be marginal - if you need to evaluate a lot of triggers and variables, it might slow down things a little. But apart from that, inserting GA via GTM is not that much different from using the snippet (basically, GTM creates a script tag).
But given that it is not broken, there is no good business case to fix it. At some point you might want to update to GA4, and then it is still time to move things to GTM.