Basically, you can hardly notice a drop in performance of such a high-end SoC. That’s unless you live to benchmark smartphones. User experience will be pretty much the same. Even huge games won’t suffer noticeably unless the underclocking is extreme which is unlikely.
There are so many variables to consider with over/under clocking. Qualcomm say that the SD 835 is optimized for better battery life which I don’t deny. This leaves Xiaomi and their hardware choices.
They could have underclocked the already optimized SoC to eek out just a bit more battery life because they think the SD 835 is TOO MUCH and would rather make it slower to conserve the small(ish) 3,350 mAh battery. (doubt that because the display eats much of any Android smartphone battery anyway)
They could also have done it because the other components of the phone eg. data buses cannot keep up with the SD 835 hence unable to fully utilize its potential (more likely.)
The underlying point of all this is, why keep it if you cannot use it?