High CPU usage with EPUB files.

```
 * Xreader version (xreader 2.4.4)
 * Distribution - (Mint 19.3 Tricia)
``` 
**Issue**
Regular xreader CPU usage with PDFs (I have i7-5500u is about 3%) but whenever I open an EPUB file it jumps to 100-130%. I see also a new process webkitwebprocess that uses 40-70% of CPU.


**Steps to reproduce**
Opening an EPUB file.


**Expected behaviour**
Regular CPU usage



