Use the -turbo command line option to have it run constantly in turbo mode. Or hold the F12 key for a temporary speed boost. shift-F12 will keep turbo mode active without having to hold F12 and will turn off when you release F12.
In order to let you experience the TRS-80 in the best light trs80gp automatically enters turbo mode when doing cassette of diskette I/O. That can be turned off with the Cassette → Auto Turbo and Diskette → Auto Turbo menus.
Normally a turbo mode would cause massive key repeats because your normal typing speed will appear to the TRS-80 as if each key has been held down for a very long time. This is mitigated by trs80gp dropping out of turbo mode whenever a key is pressed. Use Keyboard → Auto De-turbo to turn off this feature if it isn't a problem for your application. Typically games still work fine and you can challenge yourself by playing them at high speed.
At the expense of graphical and timing accuracy the emulator can run up to twice as fast as turbo mode with the -haste command line argument (or by using the File → Haste menu item). As the old saying goes, haste makes waste so only use this when speed is vital. Besides only updating the display once per second, "haste" skips wait state calculations which will throw off the timing of a program that accesses the display. Any video capture will also be distorted.