The PC slots are just a passive backplane in 8-bit (XT-mode) or 16-bit (AT-mode) and they communicate with the PC cards the same as a PC or a SBC (Single Board Computer) at a pretty crappy 4.77 MHz (XT) rate or 6 to 8 MHz (AT). Since they are passive electrical connections, the bus speed is set by the CPU board. If you recall, the EISA was an ignored attempt to go 32-bit, and IBM's own microchannel was a failed (proprietary) 32-bit attempt before the AGP and PCI came into play.
What about VESA Local Bus or more generally called VL-Bus or VLB. This was also a 32 bit extension to ISA primarily for Video Boards, but could be used for just about anything
