It may be your cardbus, but it has been noted in
official OpenBSD documentation that the older Realtek rl(4) based cards are relatively poor performers. The rl(4) man page lists three long paragraphs about poor documentation and the choices made by the driver designer, under the BUGS section.
The
Realtek Wikipedia entry quotes FreeBSD driver documentation in source code (the citation is buried in footnote #1) that calls the 8139 some very terrible things. See the section in that page under Criticism and Praise. Do note that just below the FreeBSD notation, Theo De Raadt is quoted as praising
other NICs from Realtek.