Hello
I just installed a new sink and it seems to have a really slow drain. This is how the plumbing goes. Sink to drain to 90 to 6"straight to 90 to 24" straight through floor to p trap into rest of drain system. I can not see why. All new pipes. Help anyone!

Can you post a photo of these sink drains ?

If I read your question correctly your sink has a "S" trap and is not vented. This could be the cause of your lazy drain. This wouldn't be a vessel sink in the bathroom would it? Regards, Tom