SELECT 
  pfv.feature_id, 
  pfv.product_id, 
  pfv.variant_id, 
  gp.group_id 
FROM 
  cscart_product_features_values AS pfv 
  INNER JOIN cscart_product_variation_group_products AS gp ON pfv.product_id = gp.product_id 
  INNER JOIN cscart_product_variation_group_features AS gpf ON gpf.group_id = gp.group_id 
  AND gpf.feature_id = pfv.feature_id 
WHERE 
  pfv.lang_code = 'en' 
  AND gp.group_id IN (155, 848, 845, 863, 837, 850, 860)

Query time 0.00136

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "24.71"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "gpf",
          "access_type": "range",
          "possible_keys": [
            "PRIMARY",
            "idx_group_id"
          ],
          "key": "idx_group_id",
          "used_key_parts": [
            "group_id"
          ],
          "key_length": "3",
          "rows_examined_per_scan": 9,
          "rows_produced_per_join": 9,
          "filtered": "100.00",
          "index_condition": "(`softwarepirmam_hewadelivard_cscart_4`.`gpf`.`group_id` in (155,848,845,863,837,850,860))",
          "cost_info": {
            "read_cost": "4.91",
            "eval_cost": "0.90",
            "prefix_cost": "5.81",
            "data_read_per_join": "936"
          },
          "used_columns": [
            "feature_id",
            "group_id"
          ]
        }
      },
      {
        "table": {
          "table_name": "gp",
          "access_type": "ref",
          "possible_keys": [
            "PRIMARY",
            "idx_group_id"
          ],
          "key": "idx_group_id",
          "used_key_parts": [
            "group_id"
          ],
          "key_length": "3",
          "ref": [
            "softwarepirmam_hewadelivard_cscart_4.gpf.group_id"
          ],
          "rows_examined_per_scan": 3,
          "rows_produced_per_join": 27,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "6.75",
            "eval_cost": "2.70",
            "prefix_cost": "15.26",
            "data_read_per_join": "432"
          },
          "used_columns": [
            "product_id",
            "group_id"
          ]
        }
      },
      {
        "table": {
          "table_name": "pfv",
          "access_type": "ref",
          "possible_keys": [
            "PRIMARY",
            "fl",
            "lang_code",
            "product_id",
            "fpl",
            "idx_product_feature_variant_id"
          ],
          "key": "fpl",
          "used_key_parts": [
            "feature_id",
            "product_id",
            "lang_code"
          ],
          "key_length": "12",
          "ref": [
            "softwarepirmam_hewadelivard_cscart_4.gpf.feature_id",
            "softwarepirmam_hewadelivard_cscart_4.gp.product_id",
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 27,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "6.75",
            "eval_cost": "2.70",
            "prefix_cost": "24.71",
            "data_read_per_join": "20K"
          },
          "used_columns": [
            "feature_id",
            "product_id",
            "variant_id",
            "lang_code"
          ]
        }
      }
    ]
  }
}

Result

feature_id product_id variant_id group_id
656 2227 1600 155
656 2226 2315 155
656 8703 1559 837
656 8704 1596 837
656 8705 1600 837
656 8706 1558 837
769 8760 8888 845
769 8763 8882 845
769 8764 8882 845
769 8765 8882 845
769 8766 8888 845
656 8760 2315 845
656 8763 1595 845
656 8764 1596 845
656 8765 1600 845
656 8766 1595 845
708 8784 3966 848
708 8785 3784 848
769 8815 8884 850
769 8816 8884 850
656 8815 1560 850
656 8816 2077 850
656 8519 1560 860
656 8926 1600 860
656 8601 1560 863
656 8931 2077 863
656 8932 3421 863