AMD Mobile Athlon 64 VERY slow on 6-STABLE and 7-CURRENT

[Miguel Ramos]

        This particular laptop, with a AMD Mobile Athlon 64 3000+
        processor and
        a MSI-made motherboard runs VERY SLOWLY with FreeBSD versions on
        the
        6-STABLE branch or 7-CURRENT branch.
        
        - This happens when running either the i386 architecture version
        or the amd64 architecture version. It is not specific of the
        amd64 version.
        - This happens with acpi either enabled or disabled, although
        results aren't exactly the same.
        - This happens with apic either enabled or disabled (on
        5.4-RELEASE,
        amd64 architecture, apic had to be disabled on this computer to
        avoid crash).
        - This machine is fine on 5-STABLE, including 5.5-BETA1.
        
        This is about CPU speed, it is most noted when generating a RSA
        key, for instance. I bothered use the dhrystone 2 benchmark
        (from unixbench) and the result is about 13000 dhrystones/sec
        with acpi enabled, and 38000 dhrystones/sec with acpi disabled.
        It should be above 5 million dhrystones/sec, the system is
        totally unusable, about as fast as a 386.
        
        This is not related to acpi sleep states or acpi throttle since
        it
        happens with acpi disabled.
        
        The brand of the laptop is Targa Traveller 826 (motherboard is
        MSI).
        I've tried everything I could, I couldn't yet discover some
        sysctl value that could make any difference, or even in what
        section of the kernel may the problem be.
        
        Has anyone ever had a similar problem?
        Any sugestions?
        
        I was hopping to narrow the circumstances of this problem before
        sending a pr.
        
        ----
        Miguel Ramos, miguel at anjos.strangled.net