Try setting the following in Dota’s Launch Options in steam. It disables some unused tools from the game engine, and also the background screen from the Main Menu, as mentioned above, which tends to sometimes lag some hardware for whatever reason.
Just copy&paste as is, or cut some of them down as needed. Some of them (if not all) are self explanatory on what they do.

-novid +fps_max 120 -high -map_dota -nohltv -nojoy -novr +map_enable_background_maps 0 dota_screen_shake 0 dota_reset_camera_on_spawn 0

-novid // Removes introduction video from game (Might want to disable this one whenever a new event/heroe is released, as this will skip the intro).
+fps_max ## // Max FPS the game will run at.
-High // CPU Priority, this will tell your hardware to focus on dota instead of other mundane things.
-map_dota // This will pre-load the in-game map, so loading time when joining a match will be lowered.
-nohltv // Spectator tool disabled.
-nojoy // Joystick tools disabled.
-novr // VR tools disabled
+map_enable_background_maps 0 // Disables Main Menu screen.
dota_screen_shake 0 // No camera shaking in-game.
dota_reset_camera_on_spawn 0 // Controls if camera/point of view will focus on heroe once it respawns in fountain.