You can install the old version from:
https://www.smartftp.com/support/kb/smar ... f2599.html
However the Transfer Queue will be part of version 3.0 as it has much more benefits in most situations. While I agree it's not very convenient if you are limited to one connection to the server you can still use the Transfer Queue. The other problem that it takes 15-20 seconds to connect to the server is another limitation from your server. It's most likely a problem on the server side or your overall Internet connectivity is extremely limited. It's also important to note that you will only see the connection delay for the first file/connection, afterwards the connection will be reused even if the queue is empty and you add files at later time.
I strongly believe that these limitations are not the common case and affect only a very tiny percentage of users.