Is there any significant difference between f:ajax and a4j:ajax tags?
I know about a4j:ajax from Richfaces 4 is based on native f:ajax JSF2 tag adding some attributes not found in f:ajax like onbegin, onbeforedoupdate, bypassUpdates, limitRender, status and focus but which one to use when I only need to send a simple ajax request or partial rendering ? Is there performance issues between f:ajax and a4j:ajax?
The <a4j:ajax> tag is just the more convenient form of the basic <f:ajax> tag. You can find a nice overview of all differences in this page.
The execute attribute supports an extra @region value pointing to the <a4j:region>. Any EL in execute and render attributes is resolved in current request instead of in initial request.
The JS function of the onevent attribute of both tags will be invoked 3 times (the passed-in data.status has then the values begin, complete, success). This forces you to use a switch or if when you're only interested in one of them or want to treat them differently. The <a4j:ajax> makes this more convenient with onbegin, onbeforedomupdate and oncomplete attributes respectively. See also this related question: JSF 2: How show different ajax status in same input?
If you're not interested in any of those enhancements, then using <f:ajax> should be perfectly fine as well. The performance difference is (and must be) totally negligible and not be the reason to choose one over the other.
