webgl, Unity time and system time mismatch with low FPS

Hi, I made a simple spatial memory experiment (player walk around the scene and remember the location of props. I analysed their memory error for my scientific research ) using Unity 2019.1. I built WebGL, posted on my webserver and recruited players (=experiment participants) from Mechanical Turks. The problem is that at least 15% of players experienced a laggy behaviour (low frame rate) and there is a huge mismatch between the Unity internal time and System time.

For instance, Unity.Time showed that 300 sec has passed between the events, but System.DateTime.Now indicated that 600~2000 sec has passed between the events. This problem occurred in only some players and I have no idea why or how this happened. I noticed that when this discrepancy happened, frame rate was low as 10FPS. Most other players had a decent FPS (50 or higher) and Unity Time and System Time match. I don't know what kind of system, browsers were used by players who experienced the laggy behaviour. I only have one anecdote from anonymous player that initial loading of the WebGL was fine, but the movement was very slow.

1. Does anyone experience a similar problem?

2. Why is there a discrepancy between the Unity Time and System Time? (there is no Time.scale/pause used in my game, so I think the two should be almost perfectly in line)

3. Is there any recommended practice/procedure that only allows users with reasonable computing facility to play the game?

any comments, help would be greatly appreciated! Thanks

Misun

Additional information about how I recorded the player's position in while loop (like below).

 IEnumerator familiarPhase(){
    string savefn=subId+"_familiarisation_"+System.DateTime.Now.ToString("yyyyMMdd_HHmm")+".txt";
    string savetext="";
 
    float inittime=Time.time; float timelimit=300;
    while (Time.time-inittime<timelimit){
       savetext=savetext+Time.time.ToString("F2")+"/t"+character.transform.position+"/n";
       yield return null;
    }
    StartCoroutine(save2file(savefn,savetext)); //custom made function that save the string into file on the web server (using Unity SendWebrequest)  }
 

I included System.Time information in the file name to know when the player started this part of the game. I then saved Unity internal time (Time.time). In my experiment(=game), multiple small tasks are executed in order.