[Galaxy S 5 General] crazeco:
As far as input is concerned, I'm really talking about the overall speed and ease of doing two things: Turning the device on and off and accessing basic navigation buttons. Lag is mitigated on the S5 when you disable animations and bloat (I acknowledge you should not have to do this, and I chastise Samsung for it).

There are essentially three ways to turn the M8 on:

1) Double tap if you are holding the phone;

2) Swipe the screen if you are holding the phone;

3) Power button at the top.

#1 drives you to a widget lockscreen. You choose the widget you want and then have to input your pin, unless it's the camera.

#2 will generally drive you to the pin screen, depending on which way you swipe. I used this the most often.

#3 was ergonomically very difficult to access due to both the location of the button at the top of the device and the amount it's recessed. I avoided at all costs to prevent dropping the device.

There are two ways to turn the S5 on:

1) Power button at your thumb (if right handed);

2) Home button at your thumb (if right or left handed).

Either option is extremely efficient because both buttons are located in the perfect position. I personally use #2 because I can immediately swipe with my thumb on the fingerprint scanner at the same time. After spending a week with each phone, unlocking the S5 is consistently faster than unlocking the M8. I only have to access ONE lockscreen on the S5. Everyone likes to talk about how "fast" the M8 is compared to the S5. I think you are going to notice speed on basic entry into the device far more than you notice how many nanoseconds more quickly you load Angry Birds.

There are two ways to turn both the M8 and S5 off:

1) Hit the power button;

2) Wait for the screen to turn off.

This is very important to me, because I turn my screen off EVERY SINGLE TIME I use my device. It has become a major annoyance having to use two hands to turn the device off, because I'm paranoid about dropping it when trying to one-hand push the power button.

It is VERY difficult to press the power button on the M8 with one hand. Waiting for the screen to turn off is not an acceptable option. It is very EASY to turn the screen off on the S5 with my thumb. I don't need to move the device at all.

HTC needs to implement some additional method for turning the device off. Really, just moving the power button would fix this.

Navigation buttons:

The S5 has a built in home button that is always there. There are two capacitive buttons that are always there. This has the advantage of allowing both tactile feedback and muscle memory to prevail. I don't have to look at the device to know where the buttons are.

First, the M8 has a empty row with a logo. I don't care about how this makes the device look, I do care about how it affects my ability to issue inputs into the device. I was consistently pushing on that bezel and not the screen. Second, a number of programs I use HIDE the primary buttons. You have to tap the screen in a specific way to make those buttons re-appear. There's lag time between making the buttons appear and being able to push one. The S5 does not suffer from either one of these issues, and these both affect overall performance and speed just as much as perceived "lag" from Touchwiz.

Finally, my last gripe on inputs is the location of the back button. Samsung has it right. Making me reach across the screen to hit the back button, when I use it 95% more than the recent apps button, is not acceptable.