Rene Ritchie:

Lightning has pretty much been stuck at USB2’s half a gigabit per second, since… 2012.

And:

You can now record the highest quality video of any phone on the planet, you just can’t get it off any faster than the cheapest phone on the block.

That’s the speed issue, an issue (as Rene points out) that impacts a small subset of iPhone users. But combine that with an issue that impacts a huge number of iPhone users (anyone with, say, a modern iPad): That blasted need for two different cables, Lightning for your iPhone, USB-C for your iPad.

Rene does a nice job laying all this out (watch the video embedded below). As usual, a firehose of detail, but easy to follow, especially with the edited for clarity transcript in the linked post, if reading is more your style.