I've seen the flip side of the coin in an app I inherited at work - it turns out server-side rendering can be significantly slower than client side rendering if you're rendering a lot of content, to the point where we hit an automatic timeout. If this app was a fat JS app, we probably wouldn't be trying to decommission it & it probably would load in seconds at worst, but the investment to modernize it/improve performance & scalability so it would meet our users' needs is too much.