writer highlighting tool causes high cpu use

Whenever I’m using the highlighting tool, my laptop fan goes on and the cpu usage of writer is 25%. When I hit escape to exit the tool, the cpu drop down to zero. Is this a known bug and is it fixed in later versions? I’m using what came with Linux Mint XFCE 18.2 64 bit.

Build ID: 1:5.1.6~rc2-0ubuntu1~xenial2
CPU Threads: 4; OS Version: Linux 4.8; UI Render: default;
Locale: en-US (en_US.UTF-8); Calc: group