First rule is, stay in control. If you can't take a hand off the bar and stay in control, then don't. You really need to get your skills up to that point though.

If you're still pedaling hard and taking a hand off, you probably want to not do that. For one thing, you probably don't want to be taking the corner at traffic speeds anyway, you're going to be slowing down, so you can stop pedaling. If you're NOT pedaling and you still can't maintain control with one hand, then yeah, you need to work on that. When coasting (regardless of speed) or pedaling normally, you should be able to ride indefinitely with one hand on the bar.

Also, you don't have to signal all the way through the turn. Signal BEFORE the turn, then put your hand back on the bar just before you start to turn.

By "high speed" I assume you mean 30 MPH or less. When bicycles get above 30 or so, I wouldn't take my hands off the bar anyway (I also wouldn't try to turn at those speeds).

Keeping up with traffic is nice but not if you can't do it safely. You're more likely to get seriously hurt by falling than by not keeping up with traffic. Remember, you are not required to keep up with traffic, and you're not required to get off the road if you can't. Skills first, speed later.