Sigil 1.5.1 too slow in my computer

Hello.
The new version 1.5.1 runs very slow on my computer (Windows 10 Home, Intel i5-3210M, 2.50 GHz, 6 GB RAM). It shows the changes with a lot of delay in the WYSIWYG panel. I am back to 1.4.3, which works fine for me.
Please can you tell me if there is something wrong with my computer or it is just too old?

Thanks.