Reloading a website is always initiated by the client.
All the server does is accept a request for data, check the disk for the required data, execute any necessary script processing programs, then send the result back to the client. Depending on server power, and how many people are using it for any purpose (including email, etc), this process takes normally between 100ms and about 2 seconds. Anything more than 200ms is a disaster.
Will auto refresh burden my webapp so when more than 150 members are logged in and auto refresh happens will it become unresponsive?
It will likely be unresponsive if it refreshes too fast. It will also be unresponsive if the server is unhealthy (for example, it has been hacked into).