I have installed it on a digital ocean machine. But I am not able to figure out why is it slower
What do you mean slower? Slower in downloading the page, as the initial opening?
The current meet.jit.si deployment uses cdn from aws for the web, which is available in multiple regions.
Slower as in the video conferencing is running slow not the starting page
Probably the bandwidth your server has is lower than the one on meet.jit.si. Mind also that meet.jit.si is in few regions and you are using the closest jvb available.