你正在经历的是一个Chrome bug（甚至是两个）。
基本上，当 requestAnimationFrame 回调池为空时，它们将在当前事件循环结束时直接调用它，而无需等待规范要求的实际绘制帧。
要解决这个bug，你可以保持一个不断循环的 requestAnimationFrame，但要注意这会将你的文档标记为“动画”，并会在页面上触发一系列副作用（比如每次屏幕刷新时强制重绘）。所以我不确定你在做什么，但通常这样做不是一个好主意，我宁愿邀请你只在需要的时候运行这个动画循环。

let needed = true; // set to false when you don't need the rAF loop anymore

function item_display() {
  var before = performance.now();
  requestAnimationFrame(function(timest) {
    var r_start = performance.now();
    var r_ts = timest;
    console.log("before:", before);
    console.log("RAF callback start:", r_start);
    console.log("RAF stamp:", r_ts);
    console.log("before vs. RAF callback start:", r_start - before);
    console.log("before vs. RAF stamp:", r_ts - before);
    console.log("")
    setTimeout(item_display, Math.floor(Math.random() * (1000 - 500 + 1)) + 500);
  });
}
chromeWorkaroundLoop();
item_display();

function chromeWorkaroundLoop() {
  if (needed) {
    requestAnimationFrame(chromeWorkaroundLoop);
  }
};

现在，requestAnimationFrame 回调在下一次绘制之前**触发（实际上在同一个事件循环中），并且TimeStamp参数应该表示当前帧的所有主任务和微任务执行之后的时间，然后才开始其“更新渲染”子任务（这里的步骤9）。

• [edit]：但是这并不是浏览器真正实现的，请参阅此Q/A了解更多细节。*

所以它不是最精确的，你是对的，在这个回调函数中使用performance.now()应该可以让你更接近实际的绘制时间。
此外，当Chrome在这里面临另一个bug时，可能与第一个bug有关，当他们将此rAF timeStamp设置为...我必须承认我不知道...也许是前一个画框的时间戳

(function() {
let raf_id,
  eventLoopReport = {
    id: 0,
    timeStamp: 0,
    now: 0
  },
  report = {
    nb_of_loops_between_call_and_start: -1,
    mouseClick_timeStamp: 0,
    calling_task: {
        eventLoop: null,
      now: 0
    },
    rAF_task: {
        eventLoop: null,
      now: 0,
      timeStamp: 0
    }
  };
  
startEventLoopCounter();
  
btn.onclick = triggerSingleFrame;

// increments eventLoop_id at every event loop
// (or at least every time our postMessage loop fires)
function startEventLoopCounter() {
  const channel = new MessageChannel()
  channel.port2.onmessage = e => {
    eventLoopReport.id ++;
    eventLoopReport.timeStamp = e.timeStamp;
    eventLoopReport.now = performance.now();
    channel.port1.postMessage('*');
  };
  channel.port1.postMessage('*');
}

function triggerSingleFrame(e) {
  // mouseClick Event should be generated at least the previous event loop, so its timeStamp should be in the past
    report.mouseClick_timeStamp = e.timeStamp;
    const report_calling = report.calling_task;
  report_calling.now = performance.now();
  report_calling.eventLoop = Object.assign({}, eventLoopReport);

    cancelAnimationFrame(raf_id);
  
    raf_id = requestAnimationFrame((raf_ts) => {
    const report_rAF = report.rAF_task;
        report_rAF.now = performance.now();
    report_rAF.timeStamp = raf_ts;
    report_rAF.eventLoop = Object.assign({}, eventLoopReport);
    report.nb_of_loops_between_call_and_start = report_rAF.eventLoop.id - report_calling.eventLoop.id;
    // this should always be positive
    report_el.textContent = "rAF.timeStamp - mouse_click.timeStamp: " +
            (report.rAF_task.timeStamp - report.mouseClick_timeStamp) + '\n\n' +
      // verbose
        JSON.stringify(report, null, 2) ;
  });
}
})();

<button id="btn">flash</button>
<div id="out"></div>
<pre id="report_el"></pre>

再一次，运行无限rAF循环将修复这个奇怪的错误。
因此，您可能需要检查 maybe incomingrequestPostAnimationFrame method。
在chrome:flags中启用“Experimental Web Platform features”后，您可以在Chrome 1中访问它。这个方法如果被html标准接受，将允许我们在paint操作发生后立即触发回调。
从那里，你应该在最接近的画。

var needed = true;
function item_display() {
  var before = performance.now();
  requestAnimationFrame(function() {
    requestPostAnimationFrame(function() {
      var rPAF_now = performance.now();
      console.log("before vs. rPAF now:", rPAF_now - before);
      console.log("");
      setTimeout(item_display, Math.floor(Math.random() * (1000 - 500 + 1)) + 500);
    });
  });
}

if (typeof requestPostAnimationFrame === 'function') {
  chromeWorkaroundLoop();
  item_display();
} else {
  console.error("Your browser doesn't support 'requestPostAnimationFrame' method, be sure you enabled 'Experimental Web Platform features' in chrome:flags");
}

function chromeWorkaroundLoop() {
  if (needed) {
    requestAnimationFrame(chromeWorkaroundLoop);
  }
};

对于那些还没有实现这个建议的浏览器，或者如果这个建议从来没有通过规范来实现，你可以尝试使用Task Scheduling API或MessageEvent来填充它，这应该是下一个事件循环中第一个触发的东西。

// polyfills requestPostAnimationFrame
// requestPostAnimationFrame polyfill
if (typeof requestPostAnimationFrame !== "function") {
  // Either use the Task Scheduling API if available,
  // or fallback to a MessageChannel
  const postTask = (cb) => {
    if (globalThis.scheduler?.postTask) {
      return scheduler.postTask(cb, { priority: "user-blocking" });
    }
    return new Promise((resolve, reject) => {
      const { port1, port2 } = postTask.channel ??= new MessageChannel();
      port1.addEventListener("message", () => {
        try {
          resolve(cb());
        }
        catch(err) {
          reject(err);
        }
      }, { once: true });
      port1.start();
      port2.postMessage("");
    });
  }
  const callbacks = [];
  let timestamp = 0;
  let called = false;
  let scheduled = false; // to make it work from rAF
  let inRAF = false; // to make it work from rAF

  const afterFrame = () => {
    called = false;
    const toCall = callbacks.slice();
    callbacks.length = 0;
    toCall.forEach(fn => {
      try {
        fn(timestamp);
      } catch (err) {
        reportError(err);
      }
    });
  }
  // We need to overwrite rAF to let us know we are inside an rAF callback
  // as to avoid scheduling yet an other rAF, which would be one painting frame late
  // We could have hooked an infinite loop on rAF, but this means
  // forcing the document to be animated all the time
  // which is bad for perfs
  const rAF = globalThis.requestAnimationFrame;
  globalThis.requestAnimationFrame = function(...args) {
    if (!scheduled) {
      scheduled = true;
      rAF.call(globalThis, (time) => inRAF = time);
      globalThis.requestPostAnimationFrame(() => {
        scheduled = false;
        inRAF = false;
      });
    }
    rAF.apply(globalThis, args);
  };
  globalThis.requestPostAnimationFrame = function(callback) {
    if (typeof callback !== "function") {
      throw new TypeError("Argument 1 is not callable");
    }
    callbacks.push(callback);
    if (!called) {
      if (inRAF) {
        timestamp = inRAF;
        postTask(afterFrame);
      } else {
        requestAnimationFrame((time) => {
          timestamp = time;
          postTask(afterFrame);
        });
      }
      called = true;
    }
  };
}

var needed = true;

function item_display() {
  var before = performance.now();
  requestPostAnimationFrame(function() {
    var rPAF_now = performance.now();
    console.log("before vs. rPAF now:", rPAF_now - before);
    console.log("");
    setTimeout(item_display, Math.floor(Math.random() * (1000 - 500 + 1)) + 500);
  });
}

chromeWorkaroundLoop();
item_display();

function chromeWorkaroundLoop() {
  if (needed) {
    requestAnimationFrame(chromeWorkaroundLoop);
  }
};

1.事实证明，这个功能显然已经从Chrome实验中删除了。看看the implementation issue，我不知道为什么，什么时候，也不知道他们是否计划继续工作。