They're actually the same kind of shaders. Shaders are just programs the GPU uses to determine what color should be on a particular part of a polygon*, with any GPU in the last seven years even a program that doesn't use shaders will be drawn using a default set of shaders that selects the colors in the same way that the GPU used to before shaders.
The difference between this and the shaders that your thinking of is that the ones your thinking of do more calculations with more data to make the image look better. This is what makes them slower than the default ones. This is especially true for shaders that calculate realistic lighting and shadows, as those are especially costly in both the number of calculations and the amount of data needing to be processed.
Minecraft updating to use explicit shaders will merely allow them to update to a new version of OpenGL that doesn't have to waste resources pretending to have a fixed function pipeline. It will also make modding in smarter shaders easier. It should still run on any potato released in the past seven years, at least. It will be faster in a lot of cases, and the same speed in the rest. Barring any odd bugs on a particular machine/driver.
*This has been slightly simplified.