Look & Feel:

GTK+ and Java Swing Applications do not play well at all together when it comes to look and feel. I am afraid we'd have to wait for Java 8 for a proper fix.

For the time being, if you're not using Kubuntu but Ubuntu and Unity, you have to switch to the native look and feel which is Metal.

Go to your netbeans directory (ex: /home/user/netbeans7.2/etc) and open the file : netbeans.conf

then add the following line to netbeans options:

-J-Dswing.aatext=true -J-Dawt.useSystemAAFontSettings=lcd --laf Metal

Source: http://hanynowsky.wordpress.com/2012/04/27/netbeans-ide-look-feel-under-ubuntu-12-04/

Global menus:
Java Swing applications cannot benefit from Global menus integration for now. For the same reason: GTK/Swing implementations. Even Eclipse IDE is blacklisted for now even though it's SWT and not Swing based. Same goes for Emacs and Lotus notes.
Nevertheless, you can activate Global Menu support for Java Swing applications by using Java Ayatana Library and for netbeans install the corresponding plugin : http://hanynowsky.wordpress.com/2012/05/12/integration-of-java-swing-applications-with-unity-global-menu-in-ubuntu-12-04/
By the way, it would be ideal if OpenJDK crowd include this same library in OpenJDK's next version so that Swing Application get support for Global Menu out of the box.

Slowliness:
I use Netbeans IDE frequently on my machine and it's not slow at all compared to Eclipse.
It might be an issue with the version packaged for Ubuntu Software Center (repos). I don't recommend it. Instead, download Netbeans 7.1.2 from Netbeans.org and install it locally on your home directory. You'll notice that it's faster than the version in repositories. Slowliness might also occur when you're using OpenJDK instead of Oracle JDK (Tested).