tweak integration test

tests/integration/loop-performance.tap.js

@@ -8,8 +8,26 @@
 const tap = require('tap')
 
 const TEST_DURATION = 30 * 1000
+let performanceThreshold = 0.98
+if (process.platform === 'darwin') {
+  // The GitHub macOS runners are slow and vary widely in their slowness
+  // from run to run.
+  performanceThreshold = 0.9
+} else if (process.platform === 'win32') {
+  performanceThreshold = 0.97
+}
 
 tap.test('loop performance test', { timeout: 2 * TEST_DURATION + 1000 }, function (t) {
+  // The purpose of this test is to measure how much tracking metrics
+  // impacts the processing overhead of the application loop. To do so, we:
+  //
+  // 1. Count how many times the loop fires without the native metrics
+  // hooks attached over a predetermined interval (TEST_DURATION).
+  // 2. Count how many times the loop fires with the native metrics hooks
+  // attached, and exercise the metrics by resetting them every 1 second
+  // over the test duration, again over the same predetermined interval.
+  // 3. Calculate the percentage overhead and assert it meets requirements.
+
   let timeout = null
   let callCount = 0
   let natives = null
@@ -42,8 +60,8 @@ tap.test('loop performance test', { timeout: 2 * TEST_DURATION + 1000 }, functio
 
       t.comment(noMetricsCount + ' vs ' + withMetricsCount)
       t.ok(
-        noMetricsCount * 0.98 < withMetricsCount,
-        'should not impact performance by more than 2%'
+        noMetricsCount * performanceThreshold < withMetricsCount,
+        `should not impact performance by more than ${1 - performanceThreshold}%`
       )
       t.end()
     }, TEST_DURATION)