Ever since the latest SL update, trees look like cardboard cutouts. Everything is rezzing slower.

[Condorina]

Ever since the latest SL update, trees look like cardboard cutouts, Everything is rezzing slower and lighting is too intense.

[Charolotte Caxton]

In advanced mode, go to me, preferences, graphics and adjust your settings.

[Storm Clarence]

I'm sure you have tweeked your settings in your preferences e.g., cache sizes, resolution quality, draw distance, windlight settings (default,) multi threading, etc.

If you have done all this already, then I would do a complete clean of the viewer from your machine and reinstall.  Whack all files related to SL viewer.  If you maintain chat logs, etc. on your local drive move them to a different directory/folder.  

Do a new install.

[Knowl Paine]

As mentioned, adjust your Graphic Settings in Preferences.

Your Graphic Drivers may need to be updated.

[Rolig Loon]

And if all else fails, re-install a previous version of the viewer (or at least try one of the third party viewers).