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 = 'ru' 
  AND gp.group_id IN (858)

Query time 0.00054

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "gpf",
          "access_type": "ref",
          "possible_keys": ["PRIMARY", "idx_group_id"],
          "key": "idx_group_id",
          "key_length": "3",
          "used_key_parts": ["group_id"],
          "ref": ["const"],
          "rows": 1,
          "filtered": 100
        }
      },
      {
        "table": {
          "table_name": "gp",
          "access_type": "ref",
          "possible_keys": ["PRIMARY", "idx_group_id"],
          "key": "idx_group_id",
          "key_length": "3",
          "used_key_parts": ["group_id"],
          "ref": ["const"],
          "rows": 25,
          "filtered": 100
        }
      },
      {
        "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",
          "key_length": "12",
          "used_key_parts": ["product_id", "feature_id", "lang_code"],
          "ref": [
            "lillyshop_lillyshop.gp.product_id",
            "lillyshop_lillyshop.gpf.feature_id",
            "const"
          ],
          "rows": 1,
          "filtered": 100,
          "attached_condition": "pfv.lang_code = 'ru'",
          "using_index": true
        }
      }
    ]
  }
}

Result

feature_id product_id variant_id group_id
84 12826 3101 858
84 10896 3094 858
84 10017 3092 858
84 9837 3089 858
84 9792 3087 858
84 9786 3099 858
84 9551 3096 858
84 9492 3089 858
84 8352 3090 858
84 8332 3100 858
84 8328 3104 858
84 8305 3686 858
84 7680 3101 858