I've seen that using /usr/lib/libthread_xu.so as default threading 
library gdm-binary is hungry of CPU time:

from top output:

   PID USERNAME   PRI NICE  SIZE    RES STATE    TIME   WCPU    CPU COMMAND
   750 root       152   0 12380K  4696K STOP    84:16  95.04%  95.04% 
gdm-binary

while with libc_r.so:

   811 root       152   0 10688K  3784K poll     0:00  0.00%  0.00% 
gdm-binary

 > uname -a
DragonFly lap.diff.org 1.13.0-DEVELOPMENT DragonFly 1.13.0-DEVELOPMENT 
#3: Tue Jun 17 11:26:46 CEST 2008 
fer@lap.diff.org:/usr/obj/usr/src/sys/TOSHIBA  i386

from dmesg:

TSC clock: 1496276205 Hz, i8254 clock: 1193204 Hz
CPU: Intel(R) Celeron(R) M processor         1.50GHz (1496.26-MHz 
686-class CPU)
   Origin = "GenuineIntel"  Id = 0x6d8  Stepping = 8

Features=0xafe9fbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,CLFLUSH,DTS,ACPI,MMX,FXSR,SSE,SSE2,SS,TM,PBE>
real memory  = 527302656 (514944K bytes)