RC submarines with dynamic diving systems, by the nature of how the system works, need speed in order to dive so they may tend to be faster -- and need to be faster -- than other subs. Bob at www.rc-sub.com suggests that "a good average is a fast walking pace at full throttle." If you want to increase your stealth ability -- sneaking up on unsuspecting swimmers or RC boats, then you'll want an RC submarine with a static diving system that can go very slow and surface without making a big splash.
Hobby-grade kits are upgradeable so if speed is what you want then upgrade the motor. But keep in mind that because submarines operate underwater, a sub that goes too fast could tend to get lost too soon. However, a more powerful motor may help your sub navigate stronger currents better than less powerful or stock motors.