Web browsing has always been tough on earlier iPhones due to the limited memory. Also if they're running an OS 2+ years newer than the device, it's kind of expected - the new OS has more bloat and features, and is designed mainly for newer versions of the same phone.

My iPhone OG was quite responsive until about iOS3, when it got kind of slow.

I've never had problems scrolling in non-web-based apps.

Of course, the equivalent operation on a similar-year Android or Blackberry makes you wonder why anyone bought those phones.