Why is iBooks highlighting feature so slow?

I have a macbook pro mid 2012 i56 with 4 gb ram and osx yosemite. My problem is very simple why is the iBooks highlighting feature so slow. I have even disabled sync highlights and bookmarks automatically across devices.

Slow in the sense that every highlight takes time to save and the highlighting feature is disabled for that period of time. Also there are no shortcuts and the highlight prompt that pops up after selecting the text takes time.

I use iBooks a lot while reading Epub files. Any way to sort out this problem? Thanks.