That means we technically refresh at 60/2 = 30 hz.
It thus takes 2 frames for us to finish our calculations.
Say our TV refreshes at 60 hz, but our VBlank code is longer than VBlank period. However, again, the TV determines timing, and thus you won't get another interrupt to switch back to active scan code until the gun moves back to the top of the screen, meaning an entire frame later. When your Vblank code is done, you set a flag to indicate it's done, and if that flag isn't set, rather than switching back to the active scan segment of the code to begin drawing, the system will instead return back to the Vblank code to continue running. When the raster gun moves back into position at the top of the screen to begin drawing the next frame, what happens is that the CPU of the system will switch back to a very small segment of code to check if VBlank is complete. However, since the television works at a fixed interval, if your VBlank code takes longer to execute, you'll run into problems. So Vblank is where most of the really heavy, long calculations go.
This actually in relative terms takes a very long time, and during that time the entire console's resources are free to use. Vblank is what happens when the gun reaches the bottom of the television screen and has to move all the way back to the top to begin drawing the next frame. I don't really need to explain the difference between active scan and hblank code here, but what's important is vblank code. When the gun stopped drawing and had to move to the next position, that provided perfect opportunities to run different kinds of code for different tasks not limited by BUS limitations due to scanout from the console. This is because old TVs had a physical gun that would change the position of where the scanout was being drawn. Retro consoles kinda work like this, except it's the TV that determines the switch. Since this happens so fast, we can't perceive the switch and it looks instantaneous. Every few microseconds, a CPU will switch to the next program to run. To make it look like multiple programs are running at the same time, they'll use an interrupt from the CPU to switch which program is being run. In computing, the illusion of multitasking is achieved by switching between running code - typically, a CPU can only run one program list's instructions at any given time. Think of them almost like entirely different programs. Code is split into three sections: Active scan code, HBlank code, and VBlank code. Old games work on the principle that the television itself times how code can run.
The reason slowdown occurs in retro games is because of the timing loop they operated on. There appears to be some people here confused about slowdown vs render speed here. That involves taking a donor cartridge with a higher clocked SuperFX chip and burning the Starfox rom to an eeprom then soldering it to the board.