In order to spot possible use-after-free bugs in pools, a checker has been added to OpenBSD-current, and it is enabled by default.
The drawback is a measurable general slowdown. Keep in mind that this is intentional, this is temporary and before release this should be backed out. The purpose of such a debugging tool is to get bug reports that can help a lot to get a rock solid operating system.
Benchmarks against OpenBSD-current are meaningless. If you want to do performance tests against OpenBSD, either use -stable or back out that change.