Swift performance, debug build hundreds of times slower than release

As an exercise in learning Swift, I rewrote a toy C++ command line tool in Swift. After switching to an UnsafeRawBufferPointer in a critical part of the code, the Release build of the Swift version was a little faster than the Release build of the C++ version. But the Debug build took around 700 times as long. I expect a Debug build to be somewhat slower, but by that much?

Here's the critical part of the code, a function that gets called many thousands of times. The two string parameters are always 5-letter words in plain ASCII (it's related to Wordle). By the way, if I change the loop ranges from 0..<5 to [0,1,2,3,4], then it runs about twice as fast in Debug, but twice as slow in Release.

func Score( trial: String, target: String ) -> Int
{
	var score = 0
	withUnsafeBytes(of: trial.utf8) { rawTrial in
		withUnsafeBytes(of: target.utf8) { rawTarget in
			for i in 0..<5
			{
				let trial_i = rawTrial[i];
				
				if trial_i == rawTarget[i] // strong hit
				{
					score += kStrongScore
				}
				else // check for weak hit
				{
					for j in 0..<5
					{
						if j != i
						{
							let target_j = rawTarget[j];
						
							if (trial_i == target_j) &&
								(rawTrial[j] != target_j)
							{
								score += kWeakScore
								break
							}
						}
					}
				}
			}
		}
	}
	return score
}