I must say +1 to finding a solution to this.
I rely heavily on flexible content ‘blocks’, in which each page can be customized heavily. Each block has a clone of ‘advanced settings’, which is then loaded for each flexible content block. It very quickly adds to loading and saving times.
I’d love to make more complex flexible content blocks for my projects, but I feel myself limiting options as to make it more lean. Not for the front-end, I take care of that obviously, but simply for backend loading.
Any kind of lazyload (setting), or only saving fields that have been edited would be a great addition to ACF, and open up a new range of possibilities for many of us.
