I have installed the app on two desktops (windows 10) and on two Apple products (iphone and ipad). My experience with the app is same across all these platforms - the app runs very slow and becomes non-responsive after each highlight / note / scroll command.
I’ve contacted their technical support. They only asked me to uninstall and reinstall the app, which did not solve the issue.
I tried to work in the browser version but it does not pick up the difference in highlighter colours.
It’s not that bad. I agree it is bad, but you have to just get through it and change your mindset. It does not take much to get you demotivated. Your job is to stay motivated and look at the bright side of everything. Have optimism, work hard and pass. Close the app, get back in and start reading again. It usually speeds it up. Of all the issues that you are going to have this could be the smallest.