Unknown cause of lag

I have tried on two different computers to test out the demo for Minecraft java, but for both computers there is a lot of lag. the first computer is old, so i assumed that there was lag because of the fact that it is old, but the second computer is brand new that i just got and it had the same amount of lag. I don't know if the lag is coming from the computer or from the demo itself. my new computer is fast so I don't really understand why the demo is lagging.