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Just selecting the IK Pose brush with more than one segment and/or smooth iterations heavily slows down Blender when working with high poly sculpts
Confirmed, NormalPublicKNOWN ISSUE

Description

System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce GTX 980/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 441.20

Blender Version
Broken: version: 2.83 (sub 0), branch: master, commit date: 2020-01-14 20:45, hash: rB1ec08999b6a2
Worked: (optional)

Short description of error
The way it has been explained by Pablo, this brush isn't supposed to be used on high poly sculpts because of performance issues. That I understand. However, at the moment Blender begins to freeze and slow down heavily by just selecting the IK Pose brush with segments and/or smooth interactions active, which is a bit too much. Even if you know about the slowdowns beforehand you can still easily select the brush by accident and have your project slow down to a crawl. There should be a better solution that prevents the brush from slowing down Blender when it is not in use. Just having one segment like the older version of the brush doesn't behave like this.

Exact steps for others to reproduce the error

  1. Have a fairly high poly mesh at around 1 million vertices selected in Sculpt Mode.
  2. Select the IK Pose brush with segments and/or smooth iterations activated. You should get heavy slowdowns as shown in the video below.

Event Timeline

Pablo Dobarro (pablodp606) changed the subtype of this task from "Report" to "Known Issue".Jan 16 2020, 9:36 PM

This is because in the new version calculating the preview has the same cost as starting the brush, so each time you move the mouse it will calculate the weights of 1M vertices times the number of IK segments.
There are some solutions to this, like disabling the preview automatically if the computation is going to be too heavy to be done in real time or adding the manual option to disable the preview.

@Pablo Dobarro (pablodp606) I think an automatic approach would be the best approach to this if you want the brush to be user friendly. It's a bit too much to ask new users especially to disable the preview manually to avoid this problem when they will likely do a lot of experimentation and attempts at working on higher poly meshes. Plus, it is going to get old having to disable and enable it every time you want to work with it when switching from high to low poly objects and vice versa.

That said, you could have a toggable option to always show the preview by overriding the automatic disable on higher poly meshes, but it should come with a warning about performance as a pop-up box or in the description itself.

Richard Antalik (ISS) changed the task status from Needs Triage to Confirmed.Jan 20 2020, 12:47 PM

@Pablo Dobarro (pablodp606) will confirm this on your behalf