and later in discussion Anoran mentioned Cyrillic 'Ш' and 'Щ' as 'difficult to distinguish'.
Well, I'd like to add that 'a' and 'd', as well as 'l' and 'I' and many other are tergwariously similar. But English readers usually do not have issues with distinguishing them as well as Russian readers do not get confused with 'Ш'/'Щ'.
I believe that human perception can be amazingly precise, if properly taught. But I believe that the Tengwaritis does really exist.
We are all here about constructed scripts, me included. I've produced my own writing system for Russian and exercised reading in this newly constructed script a lot (I can convert Russian texts from regular Cyrillic to my script). So I have an experience to learn reading from the beginning. There are several stages of ability to read:
1. Not all letters are memorized.
2. All letters are memorized.
3. All letters are memorized and being recalled with no effort.
4. Words are being recognized with no effort.
I have reading skill 4 in my native Russian and 3 in my conscript. In my experience Tengwaritis is a disease which happens on stages 1 and 2. So I do not think elves have any Tengwaritis except on very early stages of education.