Some random thoughts/questions about aircraft top speed limitations....

Regarding the top speeds of high speed aircraft like fighters, etc...what is the main limitation to their top speed?

Thinking about how the F-111 is such a fast airplane, does it's variable geometry wings allow it to achieve a much higher critical mach speed than other fighters? Other things not-withstanding (like frontal area of turbine engine fans, etc), how much of a factor is critical mach in affecting the top speed of fighters vs. say brute engine thrust?

Also, looking at airliners, the 727 and 747 are both very fast in their own rights, and have considerable wing sweep compared to other airliners. Coincidence?

The Hornet is certainly a fast airplane, but not nearly as fast, and also doesn't have nearly as swept back wings. Coincidence? It certainly isn't lacking in thrust and doesn't appear to have any significantly draggy aspects of its design.

Thanks for humoring me!