If two to three seconds of waiting (that's on the high end, and only for certain apps) is breaking your flow, you need to work on your attention span. That's ADHD territory.
Edit: For that matter, interacting with your phone for a few seconds at a time is probably an attention deficit issue in and of itself. I admit that it's common, but that doesn't mean it isn't a problem.
Or, any other IM program (including text messages). Or getting directions. Or checking departure time of a bus. Or switching a song that's playing. Or turning my Hue lights on/off. Or checking my account balance. Or paying with the phone.
Each of those actions should take no more than couple of seconds (occasionally, a couple dozen). And it can, if your phone is not lagging out after lockscreen or when trying to load an on-screen keyboard.
All these things work just fine on these older devices. The pauses the original message mentioned are mostly encountered when starting or switching between applications. Once the application is running things work as intended. I use Telegram on that Motorola Defy I mentioned, no problems. It runs navigation apps (Navigon, OsmAnd~) without problems. It takes photo's of reasonable quality, those photo's can be edited on the device. I use it to play music on the device itself using Dsub (a Subsonic client) or Apollo, to control remote players using MPDroid (which controls mpd (music player daemon) on remote devices). It plays video from the likes of Youtube and Vimeo just fine. I use it to read books and publications, no problems. I even use it as a telephone every now and then...
It can take a few seconds to switch between any of these apps, especially when there are several of them running in the background. Would a new device be faster? Sure it would. Will I buy a new device sometime in the future? Sure, when this device kicks the bucket or another device shows up which offers the same feature set (good performance (compared to current devices), good battery, waterproof and sorta-shockproof yet still looks like a normal device instead of some prop from a B-movie). Do I feel like I'm missing out on something by using a 6 year old phone? No, I do not.
Well, if you use a 6-years-old phone and it works for you, then by all means, stick to it. I would, too.
In my case it's not about chasing the newest features and highest resolutions - it's about certainty of getting a quality product. I no longer want to risk getting a shitty, laggy phone, or a phone that turns into one after few months. I consider it not worth the frustration it causes in daily usage.
Edit: For that matter, interacting with your phone for a few seconds at a time is probably an attention deficit issue in and of itself. I admit that it's common, but that doesn't mean it isn't a problem.