View attachment 27684
If the "Screen magnifier" is turn on, the response speed will become much slower. On other WP8 phones like 920 or 820, if you turn off the Screen magnifier, the touchscreen response speed would be OK. But it is useless on 620. Maybe the screen magnifier was turned on on phones you tested. I had tested htc 8s, which had the similar hardware. And the htc 8s is OK.
I never realized this. My Lumia 920 is "slow" to the initial scrolling gesture. There's an perceptible delay in the slide of my finger and the scrolling initiation. Once the scrolling is in motion, then everything is smooth and response. Apparently, this setting was ON. After turning it OFF, the scrolling is much better now. My brother who used my phone briefly made the same comment about the initial delay, and I told him the WP7.5 scrolling response was super smooth. It was annoyed thinking that MS need to optimize WP8 some more.