clock works slowly when I change CPU speed

I have a problem about clock when I change CPU speed (hw.acpi.cpu). 

The default hw.acpi.cpu status of my ThinkPad A22e is

    # sysctl hw.acpi.cpu

    hw.acpi.cpu.max_speed: 8
    hw.acpi.cpu.current_speed: 8
    hw.acpi.cpu.performance_speed: 8
    hw.acpi.cpu.economy_speed: 4

and, the clock value of "top" tool updates per 2 seconds normally, 
like as "01:21:45", "01:21:47", "01:21:49",...
    (because the default delay between updates is 2 seconds)

When I change CPU speed as

    sysctl -w hw.acpi.cpu.performance_speed=4

the clock value of "top" tool updates by 1, per 2 seconds, 
like as "01:22:45", "01:22:46", "01:22:47",..., but per 2 seconds. 
So, the clock of "top" tool works on half speed of the real world. 
    (I compare the clock value of "top" tool
     and the clock value of my video recorder. 
     The problem also occur when I restart "top" tool after changing CPU speed)

This problem occurs not only to using "top" tool, 
but to executing "while :; do date; sleep 1; done" on bash prompt, 
    (When CPU speed is 4, the time value updates by 1 per 2 seconds)
and to executing "kldload snd_ich" by hand after booting.
    (When CPU speed is not 8, the result of link rate is wrong. 
     The normal value is about 48000 Hz, 
     but when CPU speed is 4, it gets double value of normal link rate)

This problem did not exist when I did cvsup on Aug 2. 
This problem exists since when I did cvsup on Aug 9. 
I did cvsup on Aug 13 again, but this problem still exists. 

And, when I use backuped kernel of Aug 2, 
by changing value of "module_path" on boot prompt, 
this problem does not occur. 
    (So I do not think my ThinkPad A22e has broken)

-- 
matoba_at_st.rim.or.jp