When curl transfers data, it attempts to do that as fast as possible. It goes for both uploads and downloads. Exactly how fast that goes depends on several factors, including your computer's ability, your own network connection's bandwidth, the load on the remote server you are transferring to/from and the latency to that server. Your curl transfers are also likely to compete with other transfers on the networks the data travels over, from other users or just other apps by the same user.
In many setups, however, you can more or less saturate your own network connection with a single curl command line. If you have a 10 megabit per second connection to the Internet, chances are curl can use all of those 10 megabits to transfer data.
For most use cases, using as much bandwidth as possible is a good thing. It makes the transfer faster, it makes the curl command complete sooner and it makes the transfer use resources from the server for a shorter period of time.
Sometimes, having curl starve out other network functions on your local
network connection is inconvenient. In these situations you may want to tell
curl to slow down so that other network users get a better chance to get their
data through as well. With --limit-rate [speed] you can tell curl to not go
faster than the given number of bytes per second. The rate limit value can be
given with a letter suffix using one of K, M and G for kilobytes, megabytes
and gigabytes.
To make curl not download data any faster than 200 kilobytes per second:
curl https://example.com/ --limit-rate 200K
The given limit is the maximum average speed allowed during a period of several seconds. It means that curl might use higher transfer speeds in short bursts, but over time it averages to no more than the given rate.
curl does not know what the maximum possible speed is — it simply goes as fast as it can and is allowed. You might know your connection's maximum speed, curl does not.