I try it remove it, it was a huge resource hog. You can do everything that pagespeed does yourself, this is useful for hosts that want to standardize their shared accounts and ensure users don't do crazy things.
This is my own experience: The nginx binary turned to be 4MB instead of 700KB. Also it used 3 times more memory, that's what I mean by resource hog. In my tests, I did not notice any speed improvements... maybe because my server is pretty optimized already. I talked to @digitalpoint about this and he also said he will not use it as everything pagespeed does can be done easy, directly into XenForo. I agree with his opinion, it is pointless.
I've tried hundreds of config tweaks for it on my fairly low traffic forum and it always results in the end conclusion that it breaks parts of xenforo. Sometimes only after several days nginx uptime, sometimes only for certain users, but the cause is always ngx_pagespeed.
Might be worth another try when it comes out of beta, but for now I wouldn't recommend it with XenForo. Any other website/web app I've tried it with hasn't had any issues that weren't directly attributable to a certain filter.
As for resource usage, once the cache settles down (i.e. it stops resampling images) nginx cpu usage is only slightly higher. RAM-wise, you need to allocate a gigabyte or so of tmpfs space to it.