ScreenCapture.CaptureScreenshot skips frames

I need to get series of screenshots with constant frame rate to create video from it with FFMPEG.

I use ScreenCapture.CaptureScreenshot() method to capture screenshots in every FixedUpdate():

 void FixedUpdate()
     {
         var delta = Time.deltaTime;
 
         if (Time.realtimeSinceStartup <= 180f)
         {
             CaptureScreenshot();
         }
         else
         {
             Debug.Log("Got " + currentFrame + " screenshots");
         }
     }
 
     private void CaptureScreenshot()
     {
         var fileName = currentFrame++.ToString("000000000") + ".png";
         var filePath = "/tmp/frames/" + fileName;
 
         ScreenCapture.CaptureScreenshot(filePath);
     }

But as I can see, it skips lots of screenshots:

 $ ls -lah . | head
 -rw-r--r--     1 user  wheel   139K Jan 18 21:05 000000001.png
 -rw-r--r--     1 user  wheel   139K Jan 18 21:05 000000006.png
 -rw-r--r--     1 user  wheel   139K Jan 18 21:05 000000009.png
 -rw-r--r--     1 user  wheel   139K Jan 18 21:05 000000013.png
 -rw-r--r--     1 user  wheel   139K Jan 18 21:05 000000016.png
 -rw-r--r--     1 user  wheel   139K Jan 18 21:05 000000019.png

Note: I use FixedUpdate() because I need constant frame rate. Some performance issues are OK, it's 2D scene that is not so complex.

So, what should I do to capture "screen" contents on every call or get constant frame rate?