SELECT 
  variant_descriptions.variant 
FROM 
  cscart_product_features_values AS v 
  LEFT JOIN cscart_product_feature_variant_descriptions AS variant_descriptions ON variant_descriptions.variant_id = v.variant_id 
WHERE 
  v.product_id = 292 
  AND v.feature_id = 18

Query time 0.00077

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "v",
      "access_type": "ref",
      "possible_keys": ["PRIMARY", "fl", "product_id", "fpl"],
      "key": "PRIMARY",
      "key_length": "6",
      "used_key_parts": ["feature_id", "product_id"],
      "ref": ["const", "const"],
      "rows": 1,
      "filtered": 100,
      "using_index": true
    },
    "table": {
      "table_name": "variant_descriptions",
      "access_type": "ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["variant_id"],
      "ref": ["orbit_cscart_orbit_cscart.v.variant_id"],
      "rows": 1,
      "filtered": 100
    }
  }
}