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 (7014)

Query time 0.00103

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "45.43"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "gpf",
          "access_type": "ref",
          "possible_keys": [
            "PRIMARY",
            "idx_group_id"
          ],
          "key": "idx_group_id",
          "used_key_parts": [
            "group_id"
          ],
          "key_length": "3",
          "ref": [
            "const"
          ],
          "rows_examined_per_scan": 2,
          "rows_produced_per_join": 2,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "1.00",
            "eval_cost": "0.40",
            "prefix_cost": "1.40",
            "data_read_per_join": "208"
          },
          "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": [
            "const"
          ],
          "rows_examined_per_scan": 15,
          "rows_produced_per_join": 30,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "2.03",
            "eval_cost": "6.00",
            "prefix_cost": "9.43",
            "data_read_per_join": "480"
          },
          "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": [
            "webmarco.gpf.feature_id",
            "webmarco.gp.product_id",
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 30,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "30.00",
            "eval_cost": "6.00",
            "prefix_cost": "45.43",
            "data_read_per_join": "23K"
          },
          "used_columns": [
            "feature_id",
            "product_id",
            "variant_id",
            "lang_code"
          ]
        }
      }
    ]
  }
}

Result

feature_id product_id variant_id group_id
649 264156 62612 7014
649 264927 62608 7014
649 264928 62609 7014
649 264929 62610 7014
649 264930 62611 7014
649 264931 62608 7014
649 264932 62609 7014
649 264933 62610 7014
649 264934 62611 7014
649 264935 62612 7014
649 264950 62608 7014
649 264951 62609 7014
649 264952 62610 7014
649 264953 62611 7014
649 264954 62612 7014
882 264156 60392 7014
882 264927 60392 7014
882 264928 60392 7014
882 264929 60392 7014
882 264930 60392 7014
882 264931 60401 7014
882 264932 60401 7014
882 264933 60401 7014
882 264934 60401 7014
882 264935 60401 7014
882 264950 60412 7014
882 264951 60412 7014
882 264952 60412 7014
882 264953 60412 7014
882 264954 60412 7014