Welcome to the world of "contrast sensitivity".  When you look at the
words on the printed page or on the screen, the image of the page is
going to go through a number of surfaces on the way to your brain.

   First, the image crosses your tear layer, which loses quality when
you blink slower while reading.  Then, its through your cornea, which
is now less transparent from having all those damn birthdays.  Now,
through the anterior chamber and through the pupil, which responds to
light much slower than before.  Now the image is in the vitreous humor,
which is now acting like a bean bag full of gelatin after years of
being like water.
The image now hits your retina, and the light energy has stimulated
millions of little rods and cones, who are responding to that
stimulation by sending little frisbee-like disks from one end of
themselves to the other.  Alas, the frisbee supply is limited and the
mangled old disks just don't fly as fast as they used to.  The landing
disks create a biochemical reaction that initiates an electrical
impulse that will travel along the optic nerve to your visual cortex.
At the cortex an image will appear and your brain will interpret what
you saw and decide what it thinks it is.  ;)

Oh, yeah.  I digressed. Black letters on white backgrounds (contrast)
creates a better picture.  The more contrast you provide yourself, the
happier your visual cortex is.

======== SNIP ==========

I have floaters and  have the same problem that you describe.

Ask your eye-doctor if you have them.

    Jack