While a Raspberry Pi is generally pretty cheap, an ESP8266 is even cheaper. And when I say cheaper, I'm talking about picking up one of these microcontrollers for $5 or even less.

Slow 3.5″ Raspberry Pi LCD Hacked To 40 MHz With ESP8266 18 Comments by: Bryan Cockfield November 17, 2016 ...

As usual I’ll be using a NodeMCU board (I really did order a lot of them) which uses an ESP8266, but there’s no reason this would be difficult to achieve on an Arduino-based device.