this is a known issue. see:
https://winscp.net/forum/viewtopic.php?t=4669
https://winscp.net/forum/viewtopic.php?t=4370

in my experience, oddly (almost nonsense) the responsiveness seems to be related to the transfer rate (or something akin to the network responsiveness of the remote host):
. if i use winSCP with a gigabit LAN connection, i can see basically no crappy UI performance.
. if i use it to transfer to a machine 9 time zones away (even over 20mbit dsl), it makes me want to rake my eyes out.

(I'm using the Norton-Commander-like interface.)

I'm connecting to a very slow server, so that changing the current folder takes a long time (when it's not cached of course). During this time, I can't, say, change the local folder, or do other . Why is that?

I encounter the same problem while transferring a file; even if the transfer is in the foreground, why can't I use the UI?