Open a file manager, like dolphin or krusader
In one panel, show the target (local) directory. On the other, show the remote directory which is accessed by the fish protocol, e.g. "fish://hostname/somedir".

Choose a huge file, in my case, a .vdi file from virtualbox having 30GB in size

Select and copy it to the local panel/filesystem.

What happens:
- copy starts
- memory allocation (RAM) is increasing constantly - right from the beginning
- at some point, swap memory is allocated
- copying gets very slow (just have a look at some network monitor, in the beginning we start with 100M/sec (gigbit connection), later on, it slows down to 2M/sec and less
- the whole system feels 'sluggish' because of the ever increasing memory consumption and the need for swapping
- when you decide to stop the copying (e.g. in the kde-taskbar, press the stop button of the current copy job), then yes, copying stops.
But memory is never freed again
- you need to kill the parent-process (dolphin/krusader) to get your memory back....

Of course, copying such large files i normally do using rsync or similar - here the memory footprint is and stays very low and the network-copying speed is constant over the whole copy process.
But what is the purpose of some file-management tool if not correctly copying files?
This is also the reason for attributing the severity to 'major'