It's harder to read text when it's fuzzy and uneven. There is a feature in the settings of my reader (I have PocketBook Basic Touch) called "Complete page refresh". To get to it: Settings - Advanced Settings - Complete Page Refresh. Choose "always" option.
When we go to the next page, without a reader doing a full page refresh, the text quality is much much worse. By default, my reader does a complete page refresh every 3 pages. So I noticed that every third page is easier to read than the others. I've changed it to "always" and now my reader is usable.
Furthermore, I use DejaVu Sans font, size 29, which is very clear for me.