Your low poly 3D model is laggy usually because factors beyond polygon count—like heavy textures, complex materials, or unoptimized geometry—are straining performance.
High-resolution textures (e.g., 4K) or materials with real-time effects (such as reflections) can slow rendering even with few polygons. Unoptimized geometry—hidden faces, overlapping vertices, or too many tiny parts—also adds unnecessary load.
To fix it, try reducing texture size (use 1K instead of 4K), simplifying materials (stick to basic shaders), or cleaning up geometry (delete hidden faces, merge vertices) to lighten the render load.