I definitely wouldn't describe it that way.
It's a messy issue. In a nutshell, the Lumia
1320, 520 and 720 all use the same GPU, the Adreno 305 (as a caveat, also note that not all Adreno 305's are created equal). However, the
1320 must do much more work to achieve the same fluidity. Example:
Lumia 520 display = 480x800 = 384,000 pixels * 30 frames per second = 11,520,000
Lumia 1320 display = 720x1080 = 777,600 pixels * 30 frames per second = 23,328,000
So, a Lumia 520 must draw over 11 million pixels per second to keep your display scrolling/animating smoothly. If a Lumia 1320 is to achieve the
same fluidity however, it must draw over 23 million pixels per second, but it doesn't have any "extra horsepower" with which to achieve that task! This is a very simplistic explanation, but it's intended to be an easy to understand explanation rather than a completely accurate one.
All other 720p devices, like the Lumia 920, use an Adreno 225 GPU. The Adreno 305 is 2013's mid range GPU (
not low-end, and definitely not lower than low-end), while the Adreno 225 is 2012's high end GPU. While the Adreno 305 is the equal of the of the 225 in most disciplines, it lags behind the Adreno 225 in some places. If the OS or apps make use of those features where the 305 lags behind the 225, then we will notice a difference in rendering performance between the 1320 and any/all other WP devices with a 720p display. How
exactly this all plays together is anyone's guess, but it does appear that some of the more attentive users are noticing unappreciated differences.
As a nod to SsSgt Bruskowiz criticisms, I too am one of those people who have never personally laid eyes on a 1320, but I don't think it's necessary to hold a device to be able to judge it. IMHO online videos and some expertise in hardware/software can go a long way. At the very least I think it's safe to say the 1320 isn't the most fluid WP device out there. You might also want to take a look at this
thread.
Finally, here is a
short overview of the Adreno GPUs.