Fix and optimization of a Canvas LMS installation & ubuntu machine

We've a working installation of Canvas LMS on an Ubuntu Server 12.04 virtual machine. We're looking to a Canvas LMS expert to solve some issues. We've currently facing some big performance issues and some other minor problems: - our canvas installation is very very slow if we have more than a couple of concurrent users; it's on a server with 4gigs of RAM, so I hope the problem is not about the hardware - the SIS import feature is blocked on the "please wait, i'm importing" message. - Canvas didn't send emails for a few days (I don't know since when) and when I rebooted the server it mailed all the backlogged messages and then it stopped again to send emails. I think it's an issue related to the async jobs service, but I didn't know anything about the canvas architecture. We'll give SSH access to the server in order to complete the job.