A shell is efficient if you use it for what it has been designed for (though efficiency is rarely what you look for in a shell).
A shell is a command-line interpreter, it is designed to run commands and have them cooperate to a task.
If you want to count to 1000000000, you invoke a (one) command to count, like seq, bc, awk or python/perl... Running 1000000000 [[...]] commands and 1000000000 let commands is bound to be terribly inefficient, especially with bash which is the slowest shell of all.
In that regard, a shell will be a lot faster:
$ time sh -c 'seq 100000000' > /dev/null
sh -c 'seq 100000000' > /dev/null 0.77s user 0.03s system 99% cpu 0.805 total
$ time python -c 'i=0
> while i <= 100000000: i=i+1'
python -c 'i=0 while i <= 100000000: i=i+1' 12.12s user 0.00s system 99% cpu 12.127 total
Though of course, most of the job is done by the commands that the shell invokes, as it should be.
Now, you could of course do the same with python:
python -c '
import os
os.dup2(os.open("/dev/null", os.O_WRONLY), 1);
os.execlp("seq", "seq", "100000000")'
But that's not really how you'd do things in python as python is primarily a programming language, not a command line interpreter.
Note that you could do:
python -c 'import os; os.system("seq 100000000 > /dev/null")'
But, python would actually be calling a shell to interpret that command line!