who cares which processor is faster. you aren't buying a processor, you are buying an entire phone. that phone either works for you, or it doesn't.
there are too many other factors that decide the speed of the phone and it has NOTHING to do with benchmarks.
How much is the OS skinned?
Is it raw android?
What screen resolution?
Etc etc etc.
The Note 4, when released, was the KING of specs. and it lagged like a POS dog.
A 2014 Moto X with 1/4th the specs was 10X smoother than the Note 4.
(the Note got better, MUCH LATER, with updates)
You say that a phone "in this day and age phones should be able to to 1080p/60"
You aren't looking at a $800 top of the line phone.
You are looking at a $399 phone.
Why do you expect a $399 phone to be able to do EVERYTHING an $800 can do and match it spec for spec?
You asked what phone should I buy, but you are comparing ONE $399 phone vs your M8 that isn't all that old, but was way more expensive when it first came out.
No one can tell you what phone to buy. Get a samung S6. it does 1080p/60. and its 2560x1440 and only 5.1"
Get an iPhone 6S. it's 4.7" and does 1080p/60 (you said you didn't want a big phone)
HTC V10, Sony Z5 / compact,
The HTC V10 does 2160P and 1080p @ 30fps, but I will bet a dollar vs a pinch of crap the video will look better from the V10 vs the HTC M8 with the M8 craptastic camera.