Speed costs money. That’s why few airplanes coming out of the factory were optimized for speed—it costs staggering sums of money to certificate a production airplane and at some point, the engineers and bean counters threw up their hands and shouted, “That’s enough!”
While the cruel realities of squared functions when it comes to speed and drag mean that to go twice as fast you have to have eight times the power, the fact that few of our airplanes were optimized for speed means that there are some mods that let you bank some more speed without going bankrupt.