For VisualBoy Advance reRecord
Controller: Options > Input > Configure Controller > "Controller 1" > Speed
For VisualBoy Advance 1.8.0b
Controller: Options > Joypad > Configure > "1..." > Speed
Hotkey [Toggle]: Tools > Customize... > OptionsEmulatorSpeedupToggle
VisualBoy Advance-M
Controller: Options > Input > Set > "Config 1..." > Speed Up
Hotkey [Toggle]: Tools > Customize... > OptionsEmulatorSpeedupToggle
You must be using VisualBoy Advance-M because I just opened up the emulator, looked for the setting and found that for Speed Up the Default key is Space.
Word of advice, Play with your settings and look for your settings, I hope you know VBA has Rewind and you got to config that to...If only they put in a Right-Click > Config Hotkey for every option.