Well ... that may not be so.
Originally Sheepshaver was made for PPC, which it ran at 'native' speeds (according to both the
official Sheepshaver page, and
a copy of E-Maculation from about that time period), only doing PPC emulation 'for non-PPC platforms'. So, it should be possible to run 8.6 at a good pace, and have a viable Classic substitute for 10.5.
A Low End Mac guide from '08 said as much.
That said, it's worth noting that:
1) Sheepshaver (the x86 version, at least) has historically tended to provide a somewhat limited experience, with weak or no support for 3D graphics, among other quirks and problems.
2) There are no very recent PPC builds of Sheepshaver that I'm aware of (the focus having shifted almost exclusively to the x86 emulation scene). The choices for prebuilt binaries seem to be a somewhat unstable '14 build, or a more stable but more limited '09 build.
3) The most recent PPC E-Maculation guide says that it is "extremely CPU hungry on PPC hosts". While SS itself may run fine, that implies that SS alongside other OS X apps can be more questionable.
So, if you can run native or Classic, you probably should, but it's a viable choice in some circumstances.