SELECT 
  * 
FROM 
  cscart_product_feature_variants 
WHERE 
  variant_id IN (5908, 1280, 1198, 1199)

Query time 0.00092

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_product_feature_variants",
      "access_type": "range",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["variant_id"],
      "rows": 4,
      "filtered": 100,
      "index_condition": "cscart_product_feature_variants.variant_id in (5908,1280,1198,1199)"
    }
  }
}