Python 3.6 f-strings have been shown to be the fastest string formatting method in microbenchmarks by Python core dev Raymond Hettinger:

#Python's f-strings are amazingly fast!

f'{s} {t}' # 78.2 ns
s + ' ' + t # 104 ns
' '.join((s, t)) # 135 ns
'%s %s' % (s, t) # 188 ns
'{} {}'.format(s, t) # 283 ns
Template('$s $t').substitute(s=s, t=t) # 898 ns

— Raymond Hettinger (@raymondh) December 14, 2019

In relative speed factors, that’s:

• f-string: 1.0
• concatenate string: 1.33
• join sequence of strings: 1.73
• %s formatting operator: 2.40
• .format() method: 3.62
• Template() method: 11.48

The reason for this speedy performance was described by Python core dev Serhiy Storchaka:

1. It can calculate the length and the kind of the resulting string and allocate a memory only once.
2. It do not parse the format string at run time.
3. It do not create a tuple, all values are on the stack.
4. It do not look up an attribute and call a method.

— Serhiy Storchaka (@SerhiyStorchaka) December 15, 2019