a good example of the UI speed of the Nexus. I can tell you from personal experience, the G3 and M8 are not going to be this smooth.
Yep. In terms of smooth operation, it'll go Nexus 5 - M8 - G3. The M8 is actually not that far behind the Nexus though. HTC did a good job with the newer Sense UI (HTC's version of Android). They seem to moving towards, not away, from stock Android... to the point where it is more theme than UI overhaul. The G3's Optimus UI was also moved in that direction. I've said this before, with a few minor tweaks, the G3's UI looks pretty close to some of the Nexus 5 themes I've seen... other than the lack of the Quick Settings panel, the changes are mostly cosmetic.
I haven't found the G3's UI to be that laggy (I'm a former Nexus user by the way)... but I've tweaked a few things and replaced it's stock launcher (the homescreen interface) with the Google Now Launcher, which is what the Nexus 5 uses. The Google launcher is noticeably smoother than the stock launcher... It's at the point where it doesn't really register any lag or jitter at all, and it's only when I use my wife's stock Nexus 5 that I notice any difference. If not for that direct comparison, it wouldn't even register as lag.
By the way, Samsung's Touchwiz interface.... if you are relatively new to Android, you'll hear plenty of people gripe about the interface on those Galaxy devices. For good reason... it's a bloated mess. Samsung touches everything. Google worked extremely hard in recent versions of Android to make it smooth, efficient, clean and modern. Samsung threw all of that into the trash and mucks with every window, dialogue box button and menu and we end up with a slow, busy and cartoonishly cheap-looking interface... and it really impacts the experience. I've seen videos showing a Moto X side by side with the vastly more powerful S5 and if you didn't know better, you'd say that the Moto had the better hardware. I had a Galaxy SIII and when I loaded a custom ROM based off of stock Android (rather than Touchwiz), it was like night and day... it was a different phone.
So.. hmm... G3, M8, Nexus 5.... it depends on how you weight things.. all three devices are pretty good. The only significant outlier is the M8's camera... it's quite sub-standard and (I think) really detracts from what is otherwise an incredible device. If it had a good shooter, on par with the Nexus 5, nevermind the frontrunning shooters in the S5 and G3, it'd be tough to beat... but that camera is like a 4 megapixel boat anchor.