I think the SA-1 chip in various late-era Super Nintendo cartridges (eg. Kirby Super Star, Kirby's Dream Land 3, Super Mario RPG) would qualify.
Here's what Wikipedia has to say about it (emphasis mine):
Similar to the 5A22 CPU in the Super NES hardware, the SA1 contains a processor core based on the 65C816 with several programmable timers. The SA1 does not function as a slave CPU for the 5A22; both can interrupt each other independently.
The SA1 also features a range of enhancements over the standard 65C816:
- 10.74 MHz clock speed, compared to the 5A22's maximum of 3.58 MHz
- Faster RAM, including 2 KB of internal RAM
- Memory mapping capabilities
- Limited data storage and compression
- New DMA modes such as bitmap to bit plane transfer
- Arithmetic functions (multiplication, division, and cumulative)
- Hardware timer (either as a linear 18-bit timer, or synchronised with the PPU to generate an IRQ at a specific H/V scanline location)
- Built-in CIC lockout, for copy protection and regional marketing control
So every cartridge with an SA-1 chip essentially has an independent SoC in it, clocked at three times the speed of the SNES's main CPU.