How can I test server performance to be exact an API?
Let's say this API: https://test.com/new_task. It's a 
Run your request (or collection) in Postman
JMeter will capture the request(s) and create relevant HTTP Request sampler and HTTP Header Manager
Once done you can 
In SoapUI 5.6 I was able to use hundreds of threads. If you need more (500 , thousands or more) your computer's performance might be the limit. In that case JMeter could be a good option.