Hi there,
we aim to answer all support topics within 24hrs. Our average response is significantly quicker than that, but if you can please afford us a little patient whilst we get around to answering. thank you.
Adding any 3rd party request such as Google Analytics is going to have a negative impact on your sites performance. There isn’t anything the theme can do to change that.
I would suggest re-adding the GTM codes, give it 24hrs to make sure Google and all the other networks are up to speed , and then recheck your site.
If it is still a major performance issue then you may want to look at some running Local Analytics. There are some plugins that provide this:
PerfMatters: https://perfmatters.io/docs/local-analytics/
CAOS: https://en-gb.wordpress.org/plugins/host-analyticsjs-local/