DragonflyBSD 6.2.1 very slow on Thinkpad T460

[Fritjof]

Hi,

I have installed DragonflyBSD 6.2.1 on my Thinkpad T460 with Hammer2.
But I discoverd the following problem:
- browsing with Firefox is incredibly slow, even moving the browser window jerks
- htop(1) shows that one of four CPU cores is also always at 100%
- I found the following message in via dmesg:
  % dmesg | grep error
  dm_target_error: Successfully initialized
  WARN_ON(domain->wake_count == 0)<4>WARN_ON(domain->wake_count == 0)<4>WARN_ON(domain->wake_count == 0)error: [drm:pid233:gen8_gmch_probe] *ERROR* Can't set DMA mask/consistent mask (-5)
  i915/skl_dmc_ver1_26.bin: could not load firmware image, error 2
  error: [drm:pid922:intel_dp_aux_wait_done] *ERROR* dp aux hw did not signal timeout (has irq: 1)!
  error: [drm:pid922:intel_dp_aux_wait_done] *ERROR* dp aux hw did not signal timeout (has irq: 1)!
  error: [drm:pid922:intel_dp_aux_wait_done] *ERROR* dp aux hw did not signal timeout (has irq: 1)! 
- % dmesg | grep i915
  i915/skl_dmc_ver1_26.bin: could not load firmware image, error 2
  drm0: Failed to load DMC firmware i915/skl_dmc_ver1_26.bin. Disabling runtime power management.
  [drm] Initialized i915 1.6.0 20171023 for dev_name on minor 0
  i915: warning, request 0xfffff800ce7d8bc0 not completed

The i915 module is loaded:
% kldstat
Id Refs Address            Size     Name
 1   21 0xffffffff80200000 1ad5398  kernel
 2    1 0xffffffff81cd6000 c808     ehci.ko
 3    1 0xffffffff81ce3000 ce90     xhci.ko
 4    2 0xffffffff81cf0000 120e0    dm.ko
 5    1 0xffffffff81d03000 1be98    dm_target_crypt.ko
 6    1 0xffffffff82b1f000 1d990    if_iwm.ko
 7    1 0xffffffff82b3d000 2068e8   iwm8000Cfw.ko
 8    1 0xffffffff82d44000 1bb260   iwm8265fw.ko
 9    1 0xffffffff83800000 118000   i915.ko
10    1 0xffffffff83918000 70000    drm.ko
11    1 0xffffffff83988000 1000     iicbus.ko


I use the standard kernel and just load the iwm and i915 modules via /boot/loader.conf and /etc/rc.conf.
Does anyone has an answer or hint how to fix this?

Best,
Fritjof