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 (837)

Query time 0.00462

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "2010.58"
    },
    "nested_loop": [
      {
        "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": 127,
          "rows_produced_per_join": 127,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "15.49",
            "eval_cost": "12.70",
            "prefix_cost": "28.19",
            "data_read_per_join": "1K"
          },
          "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": "idx_product_feature_variant_id",
          "used_key_parts": [
            "product_id"
          ],
          "key_length": "3",
          "ref": [
            "gaseus.gp.product_id"
          ],
          "rows_examined_per_scan": 55,
          "rows_produced_per_join": 2982,
          "filtered": "42.69",
          "using_index": true,
          "cost_info": {
            "read_cost": "240.19",
            "eval_cost": "298.20",
            "prefix_cost": "966.88",
            "data_read_per_join": "2M"
          },
          "used_columns": [
            "feature_id",
            "product_id",
            "variant_id",
            "lang_code"
          ],
          "attached_condition": "(`gaseus`.`pfv`.`lang_code` = 'en')"
        }
      },
      {
        "table": {
          "table_name": "gpf",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY",
            "idx_group_id"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "feature_id",
            "group_id"
          ],
          "key_length": "6",
          "ref": [
            "gaseus.pfv.feature_id",
            "const"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 2982,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "745.50",
            "eval_cost": "298.20",
            "prefix_cost": "2010.58",
            "data_read_per_join": "302K"
          },
          "used_columns": [
            "feature_id",
            "group_id"
          ]
        }
      }
    ]
  }
}