The upgrade is usually measured in request latency. The larger operational change is memory: PHP 7 rewrote the internal value representation, and arrays in particular use a fraction of what they did.
$ ps -ylC php-fpm7.0 --sort:rss | awk 'NR>1 {s+=$8; n++} END {print s/n/1024 " MB avg"}'
22.4 MB avg
# the same application on 5.6 averaged 43 MBA pool sized for the old figure is now leaving half the memory unused, so pm.max_children should be recalculated after the upgrade rather than carried over. It is the cheapest capacity increase available and it is routinely missed, because nothing is broken — the server is simply running at half the concurrency it could. Measure under real traffic; at rest the difference looks much smaller than it is.