PHP - file_get_contents（）读取块随着时间的推移变慢

I'm making some tests to store large files locally with IndexedDb API, and I'm using PHP with JSON (and AJAX on Javascript's side) to receive the file data.

Actually, I'm trying to get some videos and, to do so, I use the following PHP code:

$content_ar['content'] = base64_encode(file_get_contents("../video_src.mp4", false, NULL, $bytes_from, $package_size)));
return json_encode($content_ar);

I know base64_encode will deliver 1/3 more of information than the original, but that's not the problem right now as it's the only way I know how to retrieve binary data without losing it on the way.

As you can see, I specify from which byte it has to start to read and how many of them I want to retrieve. So, on my JS side, I know how much of the file I have already stored and I ask the script to get me from actual_size to actual_size + $package_size bytes.

What I'm seeing already is that the scripts seems to run more slowly as time goes by and depending on the file size. I'm trying to understand what happens there.

I've read that file_get_contents() stores the file contents in memory, so with big files it could be a problem (that's why I'm reading it in chunks).

But seeing it gets slower with big files (and time), may it be possible that it's still storing the whole file on memory and then delivering me the chunk I tell it to? Like it loads everything and then returns the part I demand?

Or is it just storing everything until the $bytes_from + $package_size (that's why it gets slower with time, as it increases)?

If any of the above, is there any way to get it to run more efficiently and improve performance? Maybe I have to do some operations before or after to empty memory resources?

EDIT:

I've made a screenshot showing the difference (in ms) of the moment I make the call to get the file bytes I need, and the right moment when I receive the AJAX response (before I do anything with the received data, so Javascript has no impact on the performance). Here it is:

Time increases as file bytes do so.

As you can see, it's increasing with every call.

I think the problem is the time it spends to get to the initial byte I need. It does not load the whole file into memory, but it's slow until getting into the first byte to read, so as it increases the initial point, it takes more time.

EDIT 2:

Could it have something to do with the fact that I'm JSON encoding the base64 content? I've been making some performance tests and I've seen that setting $content_ar['content'] = strlen(base64_encode(file...)) is done in so much less time (when, theorically, it's doing the same work).

However, if that's the case, I still cannot understand why it increases the slowness among time. The work of encoding the same length of bytes should take the same amount of time, isn't it?

Thank you so much for your help!