I have also noticed that, but I believe it is a buggy software and I'll tell you why..
Next time you use the front cam in low light go ahead and SWITCH to Wide Selfie. You will see the lag completely dissapeares.
These are the small things that freaking annoy me
Is it that hard to fix this crap and make all of us enjoy this phone?
Compare to other flagships its **** poor and Samsung must improve it in the next update. (Considering it as a software issue )