100k+ gold in 15 minutes - Professional difficulty gold farming

Unlock Professional difficulty first by finishing the game at least once.
Go to Chapter 3-2 Execution Grounds. Proceed through it until reaching Oil Field - Dock area, i.e. the very last area of this chapter, where you have to get to Josh's boat within 2 minutes.

Simple. On Professional difficulty all money pickups are doubled in value, so the usual 3000 gold pickup we'll be farming is 6000 gold on Pro. You can still farm on Amateur, Normal, or Veteran, but it's half as effective.

1. Once at the Oil Field - Dock area, turn right.
   
2. Then turn left.
   
3. Go forward until reaching the cutscene trigger.
   
4. Skip the cutscenes. Turn around 180 degrees and go left.
   
5. The attache case with the gold is right there.
   
6. Take the gold, quit to main menu and save the progress.

As mentioned previously, on Professional it's a 6000 gold pickup. On my PC it takes around 40 seconds to go through the entire loop of Main Menu > Continue > Ready up > Loading screen > Go take the gold > Quit > Loading screen.

40 seconds per run in 15 minutes is around 22.5 runs. Rounding down to 22 x 6000 is 132.000 gold! Even if you are not speedrunning, it's still an easy 100k+ gold in just a quarter of an hour. On lower difficulties it's half, which is still pretty nice TBH.

...to have "That's Irving's boat! Let's go!" burned into your brain.