On my Intel X3100 GMA, it's still unplayable. I hit peaks of 20 FPS. The fog and particle systems really seem to bog things down, dropping the FPS to 3 areas like on the first level. Unfortunately, you can't disable them; Valve has them marked as "cheats." Never mind completely disabling the lighting system would be much more of a cheat in this game, and you can already do that.
-dxlevel 90 -w 480 -h320
Paste these options into a file. Save it as autoexec.cfg in C:\Program Files\Steam\steamapps\common\alien swarm\swarm\cfg
//Alien Swarm //Integrated graphics card config. //http://jamesgecko.com/alienswarm // Vastly improves performance of Alien Swarm on integrated graphics cards // Unfortunately, that's not saying much. Expect 20 FPS, tops. Maybe 5 FPS in areas // with lots of fog. //Misc cl_showfps 2 //0 off, 1 fps, 2 smoothed fps. asw_spinning_stim_cam 0 //disables "picture in picture" ai_expression_optimization 1 //People don't make faces when you aren't looking at them. mat_disable_ps_patch 1 mat_vignette_enable 1 r_dopixelvisibility 0 r_drawbatchdecals 0 r_drawdetailprops 0 r_fastzreject -1 //Filtering mat_disable_bloom 1 //Disables bloom mat_hdr_level 0 //Disables HDR mat_grain_enable 0 //Disable film grain mat_grain_scale_override 1 //Disables film grain. Again. mat_aaquality 0 mat_antialias 0 mat_bloom 0 mat_bloomscale 0 mat_disable_bloom 1 mat_disable_fancy_blending 1 mat_forceaniso 0 //Models r_modellodscale 0.10 //reduce quality of models r_lod 8 //Level of detail. -8 is best, 8 is worst. mat_max_worldmesh_vertices 0 //Lighting r_maxdlights 0 //maximum dynamic lights r_dynamic 0 //No dynamic lighting r_shadows 0 //No dynamic shadows r_lightinterp 0 mat_filterlightmaps 0 mat_softwarelighting 1 //1 uses CPU over gfx card. mat_disable_lightwarp 1 r_shadowrendertotexture 0 //Textures mat_compressedtextures 1 //Integrated cards are too slow for uncompressed textures mat_specular 0 //Reduce shiny mat_showlowresimage 0 //very low quality textures. mat_mipmaptextures 0 //makes textures look even worse. Character selection screen unusable. mat_fastnobump 1 mat_picmip 4 //texture detail setting mat_parallaxmap 0 //Water r_water_drawreflection 0 //Disables some water effects r_waterdrawrefraction 0 //Ditto //datacachesize 128 //Disabling fog/particles fod_enable 0 //cheat. This would really help with performance r_drawflecks 0 r_drawrain 0 r_drawparticles 0 //cheat This would really help with performance r_particle_timescale 0 //Particles aren't animated r_particle_sim_spike_threshold_ms 0 r_decals 5 //number of visible blood, bullet holes, etc. r_3dsky 0 //We never see the sky //Ragdolls g_ragdoll_maxcount 0 ragdoll_sleepaftertime 0 cl_disable_ragdolls 1 //cheat cl_ragdoll_collide 0 //Glibs from boxes and such props_break_max_pieces 0 prop_active_gib_max_fade_time 0 prop_active_gib_limit 0 //Multicore r_threaded_particles 1 //I've got a core 2 duo. Disable this stuff on a netbook. r_threaded_renderables 1 snd_mix_async 1 mat_queue_mode 2