The "catches your finger" thing may be input lag, but it much more often is amateur level coding, it seems to me.
One of the first things you learn if you try to do a simple drawing app on a slow machine (think Commodore 64 or similar early 80's 1-2MHz computer), is that if you try to process every position change, you will lag badly.
First rule of doing a decent paint app, is to decide how large deltas you can accept between each position where you actually draw, and drop events accordingly. Small delta, and you will lag; high deltas and you either need to draw lines or will get "dots" instead of a continuous line, but your lag will be limited to that of the input device. Even an early 80's home computer can give you "lag free" painting this way, at the expense of precision if you do large, very fast movements over the screen.
This is "paint app writing 101". Yet a lesson that seems to have been lost on most people writing paint apps these days, possibly because they've become accustomed to computers fast enough that they're no longer constrained by it.
I don't know about iOS, but I do know from observation that out of 30+ draw/paint apps I've tried on Android while trying to find one I'd actually be happy to use (I did not find any; I'm picky), it is obvious that this is the problem for the vast majority of them for the simple reason that there's a huge spread in observable lag for them, and so even if we assume the fastest of them are limited by input lag, the extent of the lag on the majority of them will then still be down to crappy coding.
(the quality of paint apps for Android is just beyond awful in general; don't know about iOS)
One of the first things you learn if you try to do a simple drawing app on a slow machine (think Commodore 64 or similar early 80's 1-2MHz computer), is that if you try to process every position change, you will lag badly.
First rule of doing a decent paint app, is to decide how large deltas you can accept between each position where you actually draw, and drop events accordingly. Small delta, and you will lag; high deltas and you either need to draw lines or will get "dots" instead of a continuous line, but your lag will be limited to that of the input device. Even an early 80's home computer can give you "lag free" painting this way, at the expense of precision if you do large, very fast movements over the screen.
This is "paint app writing 101". Yet a lesson that seems to have been lost on most people writing paint apps these days, possibly because they've become accustomed to computers fast enough that they're no longer constrained by it.
I don't know about iOS, but I do know from observation that out of 30+ draw/paint apps I've tried on Android while trying to find one I'd actually be happy to use (I did not find any; I'm picky), it is obvious that this is the problem for the vast majority of them for the simple reason that there's a huge spread in observable lag for them, and so even if we assume the fastest of them are limited by input lag, the extent of the lag on the majority of them will then still be down to crappy coding.
(the quality of paint apps for Android is just beyond awful in general; don't know about iOS)