I am running a F13 guest and RHEL-6 beta guest on F13 host with the cirrus vga hw qemu emulation. The X performance of the F13 guest is much slower than the RHEL-6 beta guest - so much that on F13 guest the X is hardly usable while on RHEL-6 beta guest the X is pretty responsive and can be used without problems.
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This seems to be fixed in Fedora 15.