utf8 and utf8mb4 performance difference (1 reply)

As mysql document, Performance of 4-byte UTF-8 (utf8mb4) is slower than for 3-byte UTF-8 (utf8). To avoid this penalty, continue to use utf8

I want to know what's the performance difference, how utf8mb4 slower than utf8?

Because our app sometimes will allow user input non BMP characters, so if the performance difference is only a little, is it better to set default character as utfmb4 for table when create table or just set some specific columns as utf8mb4 when create table?