Using 10-round magazines, requiring two reloads, would add about two seconds, bringing the time to fire 30 rounds to about 11 seconds. But if the shooter had more than one handgun, there would have been no practical difference between rate of fire of the Glock (or any other semi-automatic handgun) and the 30-round rifle.
In addition to the fact that limiting the capacity of magazines would have little if any practical impact on the lethality of mass gun violence, there are many other reasons why such a legal ban would be impractical, bad policy, and arguably unconstitutional.
The millions of magazines currently in existence infer that any new legal restriction would restrict only the law-abiding. This is bad policy, because civilians need high-capacity magazines as much as — and for the same reasons — as the police.