The xnf(4) driver is a Xen-specific driver. I do not know if "manual" has any impact, but I do know that OpenBSD 6.0 is using revision 1.22 of the driver, and OpenBSD 6.1, which will be released shortly, will use revision 1.54, so there are 32 patches to the driver between releases.
If you are using OpenBSD 6.0, try a -current snapshot. If this solves the performance problem, then you know the problem will be fixed with OpenBSD 6.1 when it is released.
See the commit log here:
http://cvsweb.openbsd.org/cgi-bin/cv...ev/pv/if_xnf.c