To open the console:
-Right click on portal 2
-Then go to properities and after that to Launch options
-In the Launch options put -console
-When you open the Game ,you ll see the console
-After that,put in the console: bind "key" "toggleconsole"

Commands:
r_dynamic 0
r_3dsky 0
r_drawtracers_firstperson 0
r_drawtracers 0 (needs sv_cheats 1 ,which is automaticly enabled on singleplayer,idk why)
mat_queue_mode 2 (you can also put -1)
mat_bloomscale 0
mat_blur_b 0
mat_blur_g 0
mat_blur_r 0
cl_forcepreload 1
cl_lagcompensation 1
Fps_max 0 (you can put 300,which was the default value i think)
cl_showhelp 0
cl_autohelp 0
gameinstructor_enable 0
Bind "key" "r_cleardecals" (use this commands after turrets shoot you)
Viewmodel_fov "number less than 54" (needs sv_cheats 1)
cl_aggregate_particles 0
mat_picmip 4 (sv_cheats 1)(without sv_cheats 1 ,the maximum value is 2)
r_shadows 0
r_decals 0
mat_filterlightmaps 0
mat_filtertextures 0
cl_drawhud 0 (sv_cheats 1)

Dont forget to put a 4:3 resolution and all settings in the lowest!! (800*600 best resolution!!!)

-novid
-console
+mat_motion_blur_percent_of_screen_max 0
-dxlevel 81 (if this doesnt boost fps remove this Launch option)
-threads "number of threads of your processor" (if this doesnt boost fps remove this Launch option)

Hope this worked :)
If i forget something tell It on the comments...
Give like if this guide worked :)