I've literally never read a DC comic, but what if the power you're attributing to Supes is just the power some random writer exaggerated and gave him in a random issue
You acknowledge that most writers don't give him the speed advantage you're talking about, so shouldn't we defer here to "most writers"
It's been referenced more than once but only in passive situations for obvious story telling reasons (you can't have a compelling story if Supes can literally travel as fast as flash while having super strength, heat vision, cold breath...I mean really he could deal with any situation instantly)
In the justice league movie.
Supergirl vs Flah (supergirl being a near mirror to Superman of course)
In Smallville...
In the cartoons
In the comics...mostly flash wins, some were draws Supes wins 1.
I'd say flash SHOULD be faster but the point is that supes is fast enough to cause his enemies to literally freeze in time. I refer you to a more detailed scenerio as to what happens when a super fast hero goes sonic when around non super fast heros...
Super man IS this fast. And this is what kind of freedom he would have vs the Avengers because non of them have any baseline abilities to deal with this.
But again. The comics never acknowledge this during fights or serious scenarios because it pretty much ends all drama.
So let's take this panel for instance;
It's crap. As if you remember how fast the bullets were moving in that quick silver clip, Supes would have long since caught up to the nuke before billy could whisper "shazam" let alone have the travel time for the lightening bolt to travel down and transform billy. But you know...drama and story demands you don't really acknowledge his speed in these kinds of moments because then you have no story.
And this is partly what makes dc heros bad. You reference how fast super man is but you can't keep continuity with that speed because it kills your story.
Like look at the movies. In the 80's you have a superman that can travel so fast he can turn back time;
But then in the same films he struggles to catch a rocket...
In the new movies you have Superman able to move so fast that everyone appears to be frozen in time in the clip I just showed you...but then he struggles in a fight with Batman? Really?
Continuity...can't have that with superman. So ignoring writers needing to nerf superman during various moments to try and tell a compelling story...all things being equal supes mops the floor with the avengers as NONE of them are this fast.