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 (
    6941, 7017, 7641, 8356, 7642, 7632, 8359, 
    7639, 8358, 7612, 8360, 8361, 8357, 
    7630, 8363, 8362, 7640, 8364
  )

Query time 0.00329

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "265.09"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "gp",
          "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": 78,
          "rows_produced_per_join": 78,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "16.69",
            "eval_cost": "15.60",
            "prefix_cost": "32.29",
            "data_read_per_join": "1K"
          },
          "used_columns": [
            "product_id",
            "group_id"
          ],
          "attached_condition": "(`webmarco`.`gp`.`group_id` in (6941,7017,7641,8356,7642,7632,8359,7639,8358,7612,8360,8361,8357,7630,8363,8362,7640,8364))"
        }
      },
      {
        "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": [
            "webmarco.gp.group_id"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 110,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "78.04",
            "eval_cost": "22.11",
            "prefix_cost": "132.44",
            "data_read_per_join": "11K"
          },
          "used_columns": [
            "feature_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": 110,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "110.54",
            "eval_cost": "22.11",
            "prefix_cost": "265.09",
            "data_read_per_join": "85K"
          },
          "used_columns": [
            "feature_id",
            "product_id",
            "variant_id",
            "lang_code"
          ]
        }
      }
    ]
  }
}