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Before merging the dwg drawings and working on them, explode the drawing and clean it with the "purge" itcommand.
In general, hatches can cause slowness in drawing. Replace or scale up the hatch types you use in your drawings with less dense types.
Update the graphics card drivers.
If you are using a laptop, the onboard GPU is used instead of the graphics card GPU for battery efficiency. This can cause speed and crashing problems.
If your graphics card is NVIDIA, open the NVIDIA Control Panel dialog Manage 3D Settings tab. In the Global Settings section, select the "High-performance NVIDIA processor" option asthe Preferred graphics processor.
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