If you can't comfortably look at your monitor, who cares if it has negative input lag and response time or whatever it is that you're looking for in a monitor!One last tidbit, if you have a old Macbook

so I chose 27" model knowing I was about to loose 1 inch in favor of 60Hz and a low input lag (many reviews said P2815Q has a high input lag)

Comfort is the first and foremost important aspect in considering a monitor, for me at least, because I need to be able to comfortably look at it for long periods of time before deciding on any other factors such as response time, input lag, blah blah blah.

the input lag of this monitor is about 50ms

(Compared to other IPS panel such as DELL U2412M, which only has an input lag of 17ms according to AnandTech)Conclusion:If you are a FPS gamer/photographer/graphic designer, you need to stay away from this monitor.

Don't worry about the 5ms input lag this monitor has in fast response mode, it feels lightning quick.

The response time in Dell monitors generally are always higher (8ms +) and input lag is probably a bit higher than normal anyway.

I run it in 2k mode (as I planned) because in 4k mode everything is too tiny of course, but in 2k mode I get extremely sharp text, almost as sharp as on an Apple retina display

It is not as sharp and contrasty as my other 27" IPS monitor (2560x1440) but that is mainly due to the antigloss coating, which was expected

Just as good as my 2015 Retina MB Pro screen, and if it's not the same, my eye cannot detect any difference

Great color, no noticeable bleed as of yet

Text is as crisp as on the Macbook Pro's built-in screen and color saturation looks more or less same to me