// metrics.count == 10000
var metricsByStatus: Dictionary<String, Array<HKQuantitySample>> = ["low": [], "high": [], "normal": []]

// HKQuantitySample.status:
func status(low: Double, high: Double) -> String {
    if value < low {
        return "low"
    } else if value > high {
        return "high"
    } else {
        return "normal"
    }
}

// This is taking 1 second, 100% CPU
for metric in metrics {
  metricsByStatus[metric.status(low: limits["low"], high: limits["high"])].append(metric)
}