Torrents, as far as I know, uses peers to transfer data. Data that has been cut up into chunks. It is very likely that the first set of data chunks are very available on lots and lots of peers. But other sets may not. With a small set peers (or sometimes even peer) the torrent protocol cannot retrieve whatever data you miss, the quality of the connection between you and the peer(s) also becomes an issue.
Sintel isn't that great as a story, so after people have seen it once, more often than not they do not keep the downloaded file. As a result they are not sharing and only a small set of peers remain. Sintel is also pretty old by now, that will have reduced the amount of peers even further. The bunny animation on the Blender website was more entertaining than Sintel.
Anyway: Sintel 4k with FLAC audio torrent
This does not seem to slow down after downloading 250 MB of data. But speed varies a lot. At least here in South America between 100 kbit/sec to 1,5 Mbit/sec. on my 20Mbit fiber connection.
As my torrent client I used Tixati. Don't know which client was used by the OP. My understanding is that Tixati is a very decent torrent client for its user. But it seems less than popular with people offering up their data. Just to say that torrent clients are not created equally, which can have a big impact on download speed too.
Then there is also the possibility that with torrents you get as much speed as you are willing to sacrifice from your own network connection. Or that bandwidth with which are downloading is only maintained if you are sharing one or more torrents simultaneously. Something that might surprise you as well. Torrents work better/faster when you specify a maximum download and/or upload speed in your client.
For example: say you have a 50 Mbit/sec download connection in your house. By default, torrent clients are configured to use as much bandwidth as is available at any given moment. This setting may sound logical, but most, if not all, torrent clients manage this automatic adjusting bandwidth setting poorly to very poorly.
As a rule off thumb, set in your client for max download speed about 80% of your maximum download speed, in case of a 50Mbit/sec, use 40 Mbit/sec. And around 10% of the 80% download value as upload max bandwidth, in case of max 40 Mbit/sec download, set 4 Mbit/sec as max upload.
By doing so, you'll notice that your torrent client suddenly is able to continuously pull in data at the max download speed you just configured.