Why would a C programmer use escape sequences (oct/hex) for ASCII values rather than decimal?
Follow up: does this have to do with either performance or portability?
Example:
char c = '\075';
You use octal or hexadecimal because there isn't a way to specify decimal codes inside a character literal or string literal. Octal was prevalent in PDP-11 code. These days, it probably makes more sense to use hexadecimal, though '\0' is more compact than '\x0' (so use '\0' when you null terminate a string, etc.).
Also, beware that "\x0ABad choice" doesn't have the meaning you might expect, whereas "\012007 wins" probably does. (The difference is that a hex escape runs on until it comes across a non-hex digit, whereas octal escapes stop after 3 digits at most. To get the expected result, you'd need "\x0A" "Bad choice" using 'adjacent string literal concatenation'.)
And this has nothing to do with performance and very little to do with portability. Writing '\x41' or '\101' instead of 'A' is a way of decreasing the portability and readability of your code. You should only consider using escape sequences when there isn't a better way to represent the character.