Derive an exponential value from a for loop's index

There are many possible options. The simplest is probably a quadratic like a x^2 + b x + c, where x is your proportion and a, b, c are appropriate constants. In code you would need to write this as a*x*x + b*x + c.

For true exponential functions you want k exp(l x) in code k * Math.exp(l*x).

Choosing the constants is the trickier bit. To get the same initial rates for quadratic just choose b = delayRandomiserGrowthValue and c = delayRandomiserBaseValue. You could then choose the a value depending on how much longer you the delay to be. a + b + c would give you the maximum delay.

let proportionThroughContents = CGFloat(Double(idx) / Double(allNodes.count))
let delayRandomiserBaseValue: CGFloat = 40           // c
let delayRandomiserGrowthValue: CGFloat = 60         // b
let delayRandomiserQuadraticValue: CGFloat = 60      // a
let delayRandomiserValue = UInt32(delayRandomiserBaseValue
       + delayRandomiserGrowthValue * proportionThroughContents 
       + delayRandomiserQuadraticValue * proportionThroughContents * proportionThroughContents  )

For exponential k is the initial value and l*k is initial growth rate. The final delay would be k e^l.

let proportionThroughContents = CGFloat(Double(idx) / Double(allNodes.count))
let delayRandomiserBaseValue: CGFloat = 40           // k
let delayRandomiserGrowthValue: CGFloat = 60         // 
let l: CGFloat = delayRandomiserGrowthValue / delayRandomiserBaseValue  // l
let delayRandomiserValue = UInt32(delayRandomiserBaseValue * Math.exp(l*proportionThroughContents))

With these value l = 60/40 = 1.5 and the maximum delay would be 40*exp(1.5) = 179.