Then, there is the subject of speedblitzing.
If Superman can be hurt at all, speedblitzing would kill him.
His own strength and mass and speed would work against him.
Like somebody being thrown into the ocean at super-speed. The ocean turns to concrete upon impact.
Let's say Supes is "almost" unbreakable.
If Supes hit a solid opponent at one of these super-speeds, he would burst or break on impact. It would be like having another Superman hit him with another "almost" unbreakable object at faster than light speed, with super strength. It would basically make his opponent infinitely dense.
Get it?
Unless he has some kind of force field like Juggernaut, or a power that has not been mentioned yet, every time he speedblitzed would turn him to jello.