Well, ask yourself. What does the turn coordinator tell you, Junior? And what causes the the turn not to be coordinated?

I think you know the answers, you just have to know where to look 

LOL we're gonna make you work for your answer.

Is your question about rate of turn (ie. the little airplane above the ball) or just coordinated turns?  You're kinda mixing two things there.  The turn coordinator is just the ball.  The rate of turn indicator is the little airplane with the Standard Rate tick mark and only shows roll rate and rate of turn.  To make your turn coordinated, as I'm sure you know, just "step on the ball" to get it in the center.

As for the standard rate turn at 30* bank...

What factors can effect your rate of turn?  
1. angle of bank
2. airspeed

You have one of the variables already figured out.  Get yourself setup in a constant bank turn and start out slow, and then add power.  Keep the bank (and alt. of course) constant and watch what happens to your rate of turn as your airspeed increases.  

Making a turn coordinated is more about efficiency when flying normally and about safety when climbing steeply or  turning at slow airspeed (like turns from base to final). Uncoordinated turns at higher speeds can be like a car fish-tailing around a corner or like when you turn the wheel too quickly on a slippery surface and you kinda keep going straight.

At slower speeds (or steep climbs) you can be right on the edge of a stall (or spin) and being uncoordinated just aggravates that condidtion.

A standard rate turn is more about precision. Firstly.. for known airspeeds, it's a predictable "arc". For example: Unless ATC says otherwise, and you're in crowded airspace, when they ask you to "circle right 360 degrees for spacing".. they expect you to do it at a standard rate. OR... it's like a safety limit. If you're a VFR pilot flying along and find yourself suddenly IMC (it happens).. You will turn immediately 180 degrees AT STANDARD RATE (keeps you from getting too disoriented, too quickly). And (probably most importantly), a standard rate turn is "timeable". If you lose your directional gyro and aren't really poficient at compass turning (the sim is great for practicing compass turns) you can "time your turn". Two minutes at standard rate = 360 degrees (or one minute = 180, etc.). If you're flying 090 and need/want to be flying 180.. just turn right at standard rate for 30 seconds. This comes in handy flyng a hold.. a 60 second turn on either end of the "oval".

edit: The original question   ?

Your rate of turn while in a 30 degree bank (coordinated turn) will vary with airspeed.