From Googling, I found that the basics used for troubleshooting UNIX/AIX performance issues are commands like vmstat, iostat and sar. I believe these are generic commands regardless of what UNIX flavour is in used, only difference being is the format of the output.
In a real case scenario, you will not be allowed to install anything when a customer rang wanting you to check/troubleshoot performance issues. Are these commands "sufficient" enough to check/troubleshoot performance issues?
BTW, is there any best practice specific to AIX that's known to cause performance issues?
Well it all depends on what your tracking down but AIX has many performance utilities. vmstat/iostat/uptime are great for any system if you understand the output and can give you a quick snapshot of the system. To name a few other utilities check out svmon, topas, nmon(nice but not installed by default), lsps,entstat,fileplace,filemon. There are several AIX redbooks dedicated to performance tunning and troubleshooting. Infocenter is a great reference also.
also - a great way to know if there a performance issue is to have historical data. If you dont' know what normal looks like then your job is much harder. For example you may think the server has high CPU usage but it might be acceptable and the norm for that load.