System calls in Linux are really fast. So saving "thousands" of system calls when /etc/localtime is in cache doesn't actually save that much actual CPU time.
I ran an experiment where I timed the runtime of the sample program provided in the OP, except I changed the number of calls to localtime() from ten times to a million. I then timed the difference with and without export TZ=:/etc/localhost. The net savings was .6 seconds. So for a single call to localtime(3), the net savings is 0.6 microseconds.
That's non-zero, but it's likely in the noise compared to everything else that your program might be doing.
I ran an experiment where I timed the runtime of the sample program provided in the OP, except I changed the number of calls to localtime() from ten times to a million. I then timed the difference with and without export TZ=:/etc/localhost. The net savings was .6 seconds. So for a single call to localtime(3), the net savings is 0.6 microseconds.
That's non-zero, but it's likely in the noise compared to everything else that your program might be doing.