When there's an uppercase sentence, I've found it's hard to read easily. Is it just me or do others also experience this?
Yes, the paper Reading Text from Computer Screens cites studies that found that uppercase sentences are read more slowly and with lower comprehension than sentences with normal casing (links added):
When the task involves reading continuous text from paper, however, there is generally superior performance with lowercase letters. For example, Tinker  reported that text in all uppercase letters retards reading speed by 10.2-14.2% compared with text in combined uppercase and lowercase letters. Poulton and Brown  found comprehension scores were lower for text in all uppercase letters than for the normal uppercase and lowercase combination. For rapid continuous reading, Tinker  concluded that combined upper- and lowercase English texts are read more rapidly and are preferred by readers over texts all in uppercase.