system
Given the following post, respond as if you're commenting on Reddit:
human
Why do some videos on youtube load instantly while others take fucking FOREVER?
gpt
The best explanation would probably have to be what's called cacheing (pronounched "cashing"). Imagine this:
***

You run a story with aisles and aisles of various doo-dads and what-nots, and customers come up to your desk and ask you to go fetch them a whatchamacallit. You go to the back of the store, look around a little while, find yourself a whatchamacallit, and return it to the customer.

But then you notice something funny -- most customers who come in are looking to get the exact same type of product -- the whatchamacallit. For a while you continue to go to the back of the store and look for it the same old way, and return it to the customer. But then suddenly you wise up.

If most customers are looking to grab a whatchamacallit it, you can reduce the amount of time it takes to find it and deliver it to the customer by moving it to the front desk. Now whenever someone comes in looking for a whatchamacallit, you just have to look under the desk and hand it over, which saves you tons of time!

If someone were to come in looking for a doo-dad, though, you will still have to go back into the rows of aisles upon aisles to find it.

***

Youtube hosts hundreds of millions of videos, and at any given time a customer (viewer) might want to see any one of them. Searching through all that material for the video you want to see could end up being quite a time-intensive process, so Google (who owns Youtube now) have used the same strategy as illustrated above. If a video is reasonably popular now, Youtube expects that it will be reasonably popular in the future too. Thus, it bumps the video 'forward' in the chain of servers in the backend. That way, when the inevitable flood of people come to watch a video of a baby panda sneezing, they don't have to scour their storage servers high and low for the content, they just have it on hand!

You notice the speed difference because 1) Youtube finds the content faster and can stream it to you quicker and 2) it skips a couple servers on the backend, where a video would get sent to server Z, then Y, then X ... C, B, A, then to you.

It's just Google's attempt at maximizing the viewing experience for most customers.

***

**Edit: One More Thing (as pointed out by Arcs_Of_A_Jar):**

The higher quality of the video you are watching, the more bytes of data it takes to describe the video. Since your connection can only handle so many bytes at one time, you run the risk of having data streaming to you faster than you can download it. In this case, the video will pause and buffer and try to 'catch itself up' on the incoming data.