I don’t think you have yet said what ‘slow’ means? For one of my PBX with 500+ phones, looks like takes 19 seconds to list the first page (when using the default 10 per page). Going to next page is fast, presumably as it was downloading the entire list at the time.
Offhand, using the browser developer tools looks like 13-14 seconds to get the actual data, the remaining time seconds is downloading JS and other components. If I revisit the page is slightly faster as has cached the JS files and such but still a few seconds extra from some components.