We've always had performance and response time issues with Zephyr Squad. We've raised several tickets about it and as stated in the post linked by jaugust, this has been an ongoing issue for years. We especially notice issues for 2-3 days after maintenance has been done on Jira/Zephyr but our system is always slow and it seems like small groups at a time will have issues and then suddenly those people are fine but another small group will have issues. Even when the system is behaving 'normally' for us, it's slower than all our other tools/software that we use. Our testing time has increased substantially due to Zephyr performance. My company may not be staying with Zephyr Squad long-term and this will be the main reason why...
