i think Spark 2.7.0 beta has voice chat issues reported, and i don't know the state they are in.
but to answer your question, it depends on a lot of things, server load, server resources, number of concurrent users, how active/chatty users are, your network overhead, etc.
best thing i can recommend is to troubleshoot your bottlenecks.
Start by making sure your openfire is not running out of memory, open your admin ui page --> Server tab --> Server Manager sub-tab --> Java memory. It will give you a green/red bar and a MB usage out of total MB's and a %. Make sure you are not getting too close to the max usage, otherwise you will have problems. Allocate more memory if needed. Then check your server cpu time, is it being overworked? check your disk i/o, is it too busy? network speed tests to support enough bandwidth? Basically, you will have to look until you discover what your bottlenecks are.