When loading preview, active children of php-fpm increase and site access is not possible

php : 7.2
nginx 1.14.0
nc : v 18.0.14
os : ubuntu 18.04

There are a lot of files on the server that have various extensions (jpg, png, psd, if …) for approximately 15TB.
I’m using preview settings, but if I search for files in the upper right corner and scroll down to find the file I want, I can’t access the site as active children of php-fpm increase. In this case, restarting php-fpm resolves the problem, but the root cause is not found. Please help me.