Hi,
There was this Sigil bug report but I could never reproduce the problem on Mac OSX or Linux Mint.
https://github.com/Sigil-Ebook/Sigil/issues/39
The poster suggests the slowdown is associated with simply having copied things to the clipboard. He has even posted a video which I never saw. So can anyone confirm that copying things onto the clipboard or having a full clipboard is in any way associated with this slow highlighting coming and going on Linux. He then made a bug report at Qt too.
KevinH