If you are using software mode and have 4 cores or more (this includes hyperthreaded cores, as PCSX2 takes advantage of this), set the 'extra software threads' to the amount of total threads in your CPU minus one (4 cores, threads to '3', 8 cores, threads to '7', etc.).

The 'Allow 8-bit textures' in GSdx may help with speed a bit more and maybe reduce the VRAM slowdowns.

EE Cyclerate can give speedup during the races, but may also cause slowdown to the track-selection screens where videos are shown at the background.

It mostly depends on track, the lighting and the amount of cars on screen. This game requires a ton of personal tinkering to get it to run decent.This game also has very, VERY high system requirements (expect to use at least an i5 or an FX 6300 above).

Other hacks can cause crashes or the game won't boot.

This varies in dual-core CPUs as it may increase depending on the race but may cause slowdown in others.

The MTVU hack may increase speed in quad core CPUs.

This game was released on Februto critical acclaim. It features over 700 vehicles, 51 tracks, B-Spec mode, Photo mode, and 1080i support. Gran Turismo 4 is the fourth game in the Gran Turismo series. Along with Career mode, Gran Turismo 4 highlights the new GT Photo mode and GT Online mode, which lets players around the world compete over the Internet. Test-drive new vehicles in courses located in various sites, such as Nurburgring Nordschleife, New York City, the Grand Canyon, and Tsukuba Circuit. The characteristics of each car take into account weight, speed, friction, and more to deliver realistic handling. Game description: With authentic cars, courses, and car physics, Gran Turismo 4 adds an even more realistic racing simulation to the series. Publisher(s): SCEI (JP, AS, KO), SCEA (US), SCEE (EU)

NTSC-U (Greatest Hits) cover Game general and emulation properties:

Gran turismo 4 ntsc iso