A raspberry pi can potentially be programmed to look at the GPIO ports and display them on the screen. But in order to be fast enough, the programming will have to be bare-metal C or assembly. This project aims to evaluate the limitations of doing so.
These projects have a max sampling rate of 1us (i.e 1MHz) which is not enough for most uses. I think the bottleneck might be the GPIO speed rather than the processor or software. To get higher sampling rates, the GPIO internal circuitry should be able to reliably read the incoming data. Here is an article showing the GPIO benchmark results for outputting bits: