In the last 20 years, a mainstream in Earth information and decision making has been drawn by the vision of the digital earth that calls for 3D representation, interoperability and modelling. In this context, the time dimension is essential but despite its importance, not many open standards and implementations are available. The Sensor Observation Service from the Open Geospatial Consortium is one of them and was specifically designed to collect, store and share timeseries of observations from sensors. To better understand the performance and limitation of one software implementation of this standard in real cases, this study executed a load testing of the istSOS application under a high load condition, characterized by a high number of concurrent users, in three cases mimicking existing monitoring networks. The results, in addition to providing reference values for future similar tests, show the general capacity of istSOS in meeting the INSPIRE quality of service requirements and in offering good performance with less than 500 concurrent users. When the number of concurrent users increases to 1000 and 2000, only 80% of the response times are below 30 seconds, performance that is unsatisfactory in most modern usages.