When switching on my Data East Star Wars, it takes about 3 minutes to actually boot. It starts with GI, and after 3 minutes it starts with the display, the light show and is playable. After another 2 minutes the sound starts working. I have measured the between the Ground and 5V testpoints on the power supply board and found that it is only 3.1V and slowly ramps up.

All caps look like new so they might have been replaced by the previous owner. What component is most likely causing this or should I first just replace all the caps?

Took a while and I replaced all Caps, tested them and found only one to be defective "NC" and leaking (C2) 25V 100uf, which I replaced by 50V 100uf.

All others were exactly or extremely close to their ratings (like 330uf was exactly 330uf and 1000uf was 997uf). The replacements were all like 10% under their ratings. But the machine is working great now!