While Rockstar has included a benchmark tool to measure your game’s frame-rate performance with Grand Theft Auto V, however the tool is currently busted. If you attempt to run the tool it will simply flash a few scenes and then drop you randomly into the actual game. However there is a way to run the tool.
To do this you must have already completed the prologue mission. Once you do this and have created a game save, you can just goto the settings -> advanced graphics and press “TAB”.
This will work until Rockstar patches the issue, which I’m sure they will soon.