Minecart is an entity and will be slowed down by activation range, try to ensure you are within the activation range of the minecart and test again. The config option is in spigot.yml.
spigot.yml was tested in the same state as 1.18.2. I'm checking the speed at the distance where the contents of the minecart with hopper can be seen (almost in front of me, a few blocks), so I don't think the distance is relevant.
I'm very sorry, but after receiving your reply, I rewrote the problem.
I'm sorry to be late because I rely on automatic translation for posting.
Please consider the problem again.
P.S.
Using the same Paper 1.19 server (# 61),
# 1. If you load the world used in Paper 1.18.2, the transfer speed will be faster.
# 2. In the case of a world newly created with Paper 1.19, the transfer speed will be slow.
P.S. 2.
In single play (without using a server),
# 1. In the world newly created in 1.19, the transfer speed is fast.