大量の立方体を生成するスクリプト。 1Meshなので軽いはず。

GenerateManyCube.cs

using UnityEngine;

[RequireComponent(typeof(MeshFilter))]
[RequireComponent(typeof(MeshRenderer))]
public class GenerateManyCube : MonoBehaviour
{
    [SerializeField] bool m_GenerateMeshOnUpdate = false;
    [Space]
    [SerializeField] Vector3 m_CubeScale = new Vector3(1f, 1f, 1f); // 立方体の大きさ
    [SerializeField] Vector3Int m_CubeNumbers = new Vector3Int(4, 2, 4);  // 立方体の個数
    [SerializeField] float m_CubeInterval = 1f; // 立方体の配置間隔
    [SerializeField] Vector3 m_CubeIntervals = new Vector3(1f, 1f, 1f);   // 立方体の配置間隔(xyz個別)
    private Mesh m_Mesh;

    private void Start()
    {
        m_Mesh = new Mesh();

        var meshFilter = GetComponent<MeshFilter>();
        meshFilter.mesh = m_Mesh;

        GenerateCubeMesh();
    }

    private void Update()
    {
        // 毎フレームメッシュを更新
        if (m_GenerateMeshOnUpdate)
        {
            GenerateCubeMesh();
        }
    }

    /// <summary>
    /// 立方体のメッシュ生成
    /// </summary>
    void GenerateCubeMesh()
    {
        if (m_CubeNumbers.x < 1) { m_CubeNumbers.x = 1; }
        if (m_CubeNumbers.y < 1) { m_CubeNumbers.y = 1; }
        if (m_CubeNumbers.z < 1) { m_CubeNumbers.z = 1; }

        int totalCubeNumber = m_CubeNumbers.x * m_CubeNumbers.y * m_CubeNumbers.z;

        // すべての立方体の頂点座標(vertices)の生成
        Vector3[] vertices = new Vector3[8 * totalCubeNumber];
        int vi = 0;
        Vector3 vertexOffset = new Vector3(0, 0, 0);
        for (int z = 0; z < m_CubeNumbers.z; z++)
        {
            vertexOffset.z = (z - m_CubeNumbers.z / 2f) * m_CubeInterval * m_CubeIntervals.z - m_CubeScale.z / 2f;
            for (int y = 0; y < m_CubeNumbers.y; y++)
            {
                vertexOffset.y = (y - m_CubeNumbers.y / 2f) * m_CubeInterval * m_CubeIntervals.y - m_CubeScale.y / 2f;
                for (int x = 0; x < m_CubeNumbers.x; x++)
                {
                    vertexOffset.x = (x - m_CubeNumbers.x / 2f) * m_CubeInterval * m_CubeIntervals.x - m_CubeScale.x / 2f;

                    vertices[vi++] = Vector3.Scale(new Vector3(0, 0, 0), m_CubeScale) + vertexOffset; // 0
                    vertices[vi++] = Vector3.Scale(new Vector3(0, 1, 0), m_CubeScale) + vertexOffset; // 1
                    vertices[vi++] = Vector3.Scale(new Vector3(1, 1, 0), m_CubeScale) + vertexOffset; // 2
                    vertices[vi++] = Vector3.Scale(new Vector3(1, 0, 0), m_CubeScale) + vertexOffset; // 3
                    vertices[vi++] = Vector3.Scale(new Vector3(0, 0, 1), m_CubeScale) + vertexOffset; // 4
                    vertices[vi++] = Vector3.Scale(new Vector3(0, 1, 1), m_CubeScale) + vertexOffset; // 5
                    vertices[vi++] = Vector3.Scale(new Vector3(1, 1, 1), m_CubeScale) + vertexOffset; // 6
                    vertices[vi++] = Vector3.Scale(new Vector3(1, 0, 1), m_CubeScale) + vertexOffset; // 7
                }
            }
        }

        // すべての立方体の6面の頂点インデックス(triangles)を作成
        int[] triangles = new int[6 * 6 * totalCubeNumber];
        int ti = 0;
        int triangleOffset = 0;
        for (int i = 0; i < totalCubeNumber; i++)
        {
            // xy平面に平行な面
            triangles[ti++] = 0 + triangleOffset;
            triangles[ti++] = 1 + triangleOffset;
            triangles[ti++] = 2 + triangleOffset;
            triangles[ti++] = 0 + triangleOffset;
            triangles[ti++] = 2 + triangleOffset;
            triangles[ti++] = 3 + triangleOffset;

            triangles[ti++] = 6 + triangleOffset;
            triangles[ti++] = 5 + triangleOffset;
            triangles[ti++] = 4 + triangleOffset;
            triangles[ti++] = 7 + triangleOffset;
            triangles[ti++] = 6 + triangleOffset;
            triangles[ti++] = 4 + triangleOffset;

            // yz平面に平行な面
            triangles[ti++] = 4 + triangleOffset;
            triangles[ti++] = 5 + triangleOffset;
            triangles[ti++] = 1 + triangleOffset;
            triangles[ti++] = 4 + triangleOffset;
            triangles[ti++] = 1 + triangleOffset;
            triangles[ti++] = 0 + triangleOffset;

            triangles[ti++] = 2 + triangleOffset;
            triangles[ti++] = 6 + triangleOffset;
            triangles[ti++] = 7 + triangleOffset;
            triangles[ti++] = 3 + triangleOffset;
            triangles[ti++] = 2 + triangleOffset;
            triangles[ti++] = 7 + triangleOffset;


            // zx平面に平行な面
            triangles[ti++] = 5 + triangleOffset;
            triangles[ti++] = 6 + triangleOffset;
            triangles[ti++] = 2 + triangleOffset;
            triangles[ti++] = 5 + triangleOffset;
            triangles[ti++] = 2 + triangleOffset;
            triangles[ti++] = 1 + triangleOffset;

            triangles[ti++] = 3 + triangleOffset;
            triangles[ti++] = 7 + triangleOffset;
            triangles[ti++] = 4 + triangleOffset;
            triangles[ti++] = 4 + triangleOffset;
            triangles[ti++] = 0 + triangleOffset;
            triangles[ti++] = 3 + triangleOffset;

            triangleOffset += 8;
        }
        
        // メッシュ更新
        m_Mesh.vertices = vertices;
        m_Mesh.triangles = triangles;
    }
}