A few solutions for this for Windows users, with separate solutions for SketchUp 2017 and SketchUp 2018:
For 2017: This is often a computer issue caused by a faulty Windows Update that messed with the graphics card settings. So far most people have been able to fix the problem with the steps below:
#1 Fix: (easier than #2, but only fixes it 20% of the time) If you have multiple video cards, you may need to set which one SketchUp uses, please follow the steps found here: https://www.howtogeek.com/175281/how-to-force-graphics-options-in-pc-games-with-nvidia-amd-or-intel-graphics/
Now try SketchUp again, and if that didnt fix it, try #2:
#2 Fix (which more frequently fixes it): Update your video card driver using the steps below.
A. If there’s an Intel graphics card (pretty much always), follow this link for instructions: Updating Intel Driver - Google Slides.
B. If it’s not Intel:
For SketchUp 2018:
With SketchUp Pro 2018, we have a way to fix the issue with the Select tool caused by the recent Windows Update. Using the link below, you’ll find a script that helps correct the slow cursor issue that you’ve come across. Please note that this is set at the User Account level so it will need to be set for every user on a given computer. Please fill out this form so we can gather more information about this issue. The fix is at the end of the form: SketchUp Selection Lag in Windows 10
(that fix only works for 2018, not 2017)