Playing on a laptop, and I'm getting a little bit of lag when I shoot a portal up close. I'm not familiar with all the "filtering" and such, and was curious as to how I can obtain the lowest possible settings.

Pretty much just go through all the settings and change them to lower options (normally they're rather self-explanatory).

A suggestion: First, see how the anti-aliasing setting affects gameplay. In case you don't know, anti-aliasing blends edges so that when something's on an angle you don't get the "stair-step" look. My laptop can't run any anti-aliasing at all; as soon as I turn it on I start lagging significantly. That's usually a big culprit.

That being said, how's the rest of the game running? Even when I've got a satisfactory number of frames per second (which isn't often), getting close to a portal or even having one in the field of view can bring it down. It seems like that's common for a lot of different systems.