To reduce lag and improve performance in Dota 2 on a low-spec PC, disable most advanced graphics settings in the game's video options, such as anti-aliasing, world lighting, and high-quality water. Additionally, set your game's texture, effects, and shadow quality to low, lower your in-game render quality, and close unnecessary background applications. Finally, enable Windows Game Mode and set your power plan to "Best Performance".

In-Game Settings
1. Enable Advanced Video Settings: In the video settings, check the "Advanced" box to access more customization options.
2. Disable Visual Effects: Uncheck most advanced options including:
Animate Portrait
Additive Light Pass
World Lighting
Ambient Occlusion
Normal Maps
Ambient Creatures
Ambient Cloth Simulation
Grass
Anti-Aliasing
Specular & Light Blooms
High-Quality Water
Atmospheric Fog
Tree Wind
Compute Shaders
3. Lower Texture, Effects, and Shadow Quality: Set these options to the lowest possible setting.
4. Reduce Render Quality: Lower the "Game Screen Render Quality" to around 40% for a significant performance boost.
5. Set Maximum FPS: Adjust the "Maximum Frames per second" for both the game and dashboard to a reasonable value, like 240.
6. Disable Vertical Sync: Turning off vertical sync (VSync) can help reduce lag by allowing the game to render frames as fast as possible.
Windows and System Optimization
1. Close Background Applications: Ensure no other resource-intensive applications or tabs are open while playing Dota 2.
2. Enable Game Mode: Turn on Windows Game Mode by going to Settings > Gaming > Game Mode and toggling it on.
3. Set Power Plan to Performance: Go to Windows settings, search for "advanced system settings," and select "Adjust for best performance" under the Performance options.
4. Update Graphics Drivers: Make sure your graphics drivers are up to date to ensure optimal performance.
5. Use Launch Options: Add the following launch parameters in your Steam library for Dota 2 to potentially improve performance: -console -gl -prewarm -map dota +fps_max 244.
Additional Tips
Consider Hardware: If possible, upgrading your GPU or CPU can provide a substantial performance increase.
Monitor Other Issues: Check for server-side problems or high ping spikes, as sometimes lag can be due to network issues rather than your PC.