Adaptivity balances the calculations across the image, to focus more processing on tricky areas, normally places like shadows, to reduce those times that you find yourself with a clean image in the "easy" areas but still needing more render passes as you wait for one problem area to clean up. The option is tucked away as normally no reason to turn that off - but you can give it a go for testing (the new A/B comparison in the VFB history, plus render regions, is a great way to quickly compare differences in different settings).
