My Nexus 7 became slower after the update to 4.2. How can I fix it? After the update to android 4.2 my nexus 7 became slower.

Switching applications takes some seconds and even the menu that pop-ups after I push the 'share' button takes a while to appear.

Is there something I can do to have its performance back to the usual level?
As with all updates, the first thing that comes to mind is factory resetting the device. This clears all the remnants of the old version out of the system and resets app data that could cause inconsistencies. The downside is you'll lose all your apps and their data.

Normally I only do a factory reset in between major versions (3.2->4.0, 4.0.2->4.1 etc.), but if minor updates cause trouble, this would be a first step for me.

Oh yeah, the reset can be found in Settings -&gt; Backup &amp; reset -&gt; Factory data reset. Note that while this usually doesn't affect your sdcard (including internal storage), some devices may wipe that too. But not the Nexus series, so you're safe.