Okay! After some tests I found a workaround!
Please, be careful since I am not a programmer so I do not guarantee this is a good way to solve the problem in a production environment!
To solve the slowness problem (if you use Courier-IMAP as IMAP server, like me), make a backup copy of the file imap.inc (in (roundcube folder)/program/lib/).
Then, download Roundcube 0.1-rc1, open the file imap.inc near line 196 and copy the function iil_C_Authenticate.
Finally open the imap.inc file of the actual version and paste this function in order to sobstitute the same function in it (near line 254).
Works absolutely fine for me! Hope this helps you, too.
Luk