Here are a couple of optimizations you can apply you speed up things. Just thinking out loud.
Since the number of rows can be in the millions, you will want a caching system just for the JSON data from the server. I can't imagine anybody wanting to download all X million items, but if they did, it would be a problem. This little test on Chrome for an array on 20M+ integers crashes on my machine constantly.
var data = [];
for(var i = 0; i < 20000000; i++) {
data.push(i);
}
console.log(data.length);
You could use LRU or some other caching algorithm and have an upper bound on how much data you're willing to cache.
For the table cells themselves, I think constructing/destroying DOM nodes can be expensive. Instead, you could just pre-define X number of cells, and whenever the user scrolls to a new position, inject the JSON data into these cells. The scrollbar would virtually have no direct relationship to how much space (height) is required to represent the entire dataset. You could arbitrarily set the table container's height, say 5000px, and map that to the total number of rows. For example, if the containers height is 5000px and there are a total of 10M rows, then the starting row ≈ (scroll.top/5000) * 10M
where scroll.top
represents the scroll distance from the top of the container. Small demo here.
To detect when to request more data, ideally an object should act as a mediator that listens to scroll events. This object keeps track of how fast the user is scrolling, and when it looks like the user is slowing down or has completely stopped, makes a data request for the corresponding rows. Retrieving data in this fashion means your data is going to be fragmented, so the cache should be designed with that in mind.
Also the browser limits on maximum outgoing connections can play an important part. A user may scroll to a certain position which will fire an AJAX request, but before that finishes the user can scroll to some other portion. If the server is not responsive enough the requests would get queued up and the application will look unresponsive. You could use a request manager through which all requests are routed, and it can cancel pending requests to make space.