Performance issue when using Windows server 2012 and SQL 2012 express and you want to connect to a database from a workstation
Go to Start > Control Panel
Go to Windows Firewall
Click on Allow an app or feature through Windows Firewall
Allow an another app. > search for the location where you installed the MS SQL 2012 express installation. Select and click OK.
In this occasion
D:\Program Files\Microsoft SQL Server\MSSQL11.SNELSTART\MSSQL\Binn\sqlservr.exe Allow the program for Domain, Private and Public.
If this is not enough you have to add more ports to the firewall. You can follow the following URL:
http://support.microsoft.com/kb/968872