It takes time to resolve symlink. WinSCP does it, so it knows if the sylink points to file or directory. You may disable resolving symlinks or Directories tab of Login window. But if your symlink points to directory, you will not be able to enter it by clicking on it. Of course, you'll be still able to enter the directory manually (using Open Directory command).

It takes about 100 times longer to render a directory full of Sym Links than one with files. Is there any particular reason for this?

I have a directory with about 150 sym links and comparing with a directory with 150 files.

Currently using version 3.6.8, same problem existed in previous version as well.