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 = 'ja' 
  AND gp.group_id IN (10)

Query time 0.00009

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "gpf",
      "access_type": "index",
      "possible_keys": ["PRIMARY", "idx_group_id"],
      "key": "PRIMARY",
      "key_length": "6",
      "used_key_parts": ["feature_id", "group_id"],
      "rows": 2,
      "filtered": 100,
      "attached_condition": "gpf.group_id = 10",
      "using_index": true
    },
    "table": {
      "table_name": "pfv",
      "access_type": "ref",
      "possible_keys": [
        "PRIMARY",
        "fl",
        "lang_code",
        "product_id",
        "fpl",
        "idx_product_feature_variant_id"
      ],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["feature_id"],
      "ref": ["smscrm_edev.gpf.feature_id"],
      "rows": 3,
      "filtered": 93.83753204,
      "attached_condition": "pfv.lang_code = 'ja'",
      "using_index": true
    },
    "table": {
      "table_name": "gp",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY", "idx_group_id"],
      "key": "PRIMARY",
      "key_length": "6",
      "used_key_parts": ["product_id", "group_id"],
      "ref": ["smscrm_edev.pfv.product_id", "const"],
      "rows": 1,
      "filtered": 100,
      "using_index": true
    }
  }
}