So firstly, I'm shocked that Google Optimize doesn't integrate properly with GTM, it requires you manually add code before the GTM tag.
So be it, BUT:
The redirect code is a LOT worse than the Google Analytics Experiments code. It's shocking! It waits for an entire page load before finally redirecting people to the experiment version of the site. Yes, yes, I've implemented the 'page hide' so you don't see a flicker, you just see nothing for a good 1 to 2 seconds before it decides it will redirect you to the optimise experiment.
I can confirm this did NOT happen with Google Experiments.
I assume Optimize works a little better with the on-page changes since it doesn't have to redirect people? I'm super sceptical. What a stuff up for a potentially great product.
Has anyone else experienced this too?