One big, long-standing issue is that fish can't run builtins, blocks or functions in the background or at the same time.
That means a pipeline like
seq 1 5 | while read -l line
echo line; sleep 0.1;
end | while read -l line
echo line; sleep 0.1
end
will have to wait for the first while loop to complete, which takes 0.5s, and then run the second.
So it takes 0.5s until you get the first output and a full second until you get all of it.
Making this concurrent means you get the first line immediately and all of it in 0.5s.
While this is an egregious example, it makes all builtin | builtin pipelines slower.
Other shells solve this via subshells - they fork off a process for the middle part of the pipeline at least. That has some downsides in that it's annoyingly leaky - you can't set variables or create a background job in those sections and then wait for them outside, because it's a new process and so the outer shell never sees them.
Now why exactly would Gleason have issues with blocking?
It's because he's a fucking card-carrying transphobe, and, like the article says, former CTO for truth.social, Trump's social network.
Nobody should listen to anything this clown has to say.