UTF-16 is much faster in many situations than UTF-8. It really depends a lot on just what you are doing, and the data you are processing. If it is mainly in North/South America, Western Europe, or Australia/NZ, UTF-8 does OK. UTF-8 is great for data interchange, but can really slow things down if you have many non-ASCII characters (as well as bloat the size of any buffers you need - because you'll need to allocate 50% more space than for UTF-16, to be sure you can hold the same # of characters).
UTF-16 is used by Windows APIs, but also ICU, Java, C++ UnicodeString. Python 3 actually picks a 1,2,4 byte representation depending on what characters are in the string (so UTF-16, but with no surrogate pairs, when there are any characters > 0xff, but none > 0xffff). Scott
