First things first, "not slow" is subjective. To some people, if it's staying below 60 FPS a lot, it's not fast enough, and to others, a consistent 30 FPS would be fine.
Shaders may vary from one to the next (Sonic Ether's may run different to BSL which may run different to Sildur's which may run different to Continuum, and so on), from version to version (atop Minecraft itself causing the same sometimes from version to version), and even one setting to the next within the same shader (many settings can be turned lower or even off with no real image quality detriment while saving a lot of performance, and others are preference [bloom levels, lens flares, depth of field, anti-aliasing, etc.]). It's really hard to even begin to give a ballpark estimate to something like this.
Keep in mind that shaders mostly add to the GPU demands. You need a decent enough CPU for the underlying Minecraft itself as well if the goal is to prevent slowness. Without a decent (ideally desktop and higher clock speed) CPU, you'll be wanting to restrict the render distance too (and this pretty much becomes mandatory once shaders are added, it seems), as well as make sure a few key settings are set to prevent performance drops (again, render distance probably being THE key one).
That said, I get pretty smooth performance on 1.16 with OptiFine and the BSL shaders on my GeForce GTX 1060 6 GB, and that's like a 4 year old mid-range GPU. Same goes for Sildurs on the higher presets which I think is more demanding but I just prefer BSL. I have to put my render distance to 16 to 20 or so instead of higher, but it's smooth. I imagine you can certainly get by with less. I used to run Sonic Ethers on an old Minecraft version on a GeForce GTX 560 Ti (I elected not to play with them at the time, but they were "playable" at certain settings).