What kind of performance gain will I get from ditching Apache for NGINX if I have a very low traffic web site (e.g. 1000 unique visitors a day, approx 5 requests/sec at highest load, and approx 50 MB of traffic per day since lots of photos are being displayed).
Specifically, what gains (if any) would I have for:
Again, this is for a low traffic web site and I'm running on a VPS.
If you have such a low traffic, I am not sure you need to go through the troubles of changing your webserver : kind of looks like "premature optimisation" to me.
Well, at least, if those 1,000 visitors don't visit too many pages, and don't all arrive at exactly the same time.
You'd probably have way better gains for your users (and that's what matter !) by activating gzip compression for JS/CSS/HTML, and/or regrouping JS/CSS files into one instead of several, for instance.
About that, running yslow on your webite, and following some of the advices it'll give you, will probably bring more speed to your users than changing server.
Just to make clear : I don't say that you shouldn't optimize your server -- but that, with such a low traffic, it might be more interesting to display pages faster ; at least, first.
Is your Apache server taking too much CPU or RAM? I switched from Apache to Nginx to save memory, especially to serve static file: I seem to be using about 75% less memory with Nginx.
Like the other comment said, are you sure that Apache is the bottle neck? If you are not swapping, then you have enough memory. I don't think you will save any significant server side latency.