Slow file scanning + high volume load

Hello,

for some time now I’m trying to narrow down what might be causing a slow file scanning performance I’m seeing.

I’m running WordPress with Wordfence on a current Synology DiskStation 716+II. The hosted website is pretty small, but the WordPress installation is currently about 18k files. Website is run on nginx, with PHP 5.6 (php56_fpm)

Issue: Wordfence requires about 10min duration to scan about 1.000 files. So I recently reached the scan time limit of 3h and had to exclude directories to get a lower amount of files to scan.

It should be noted, that CPU usage is pretty low, but the file-system IO load goes up to 100% during Wordfence scans!!!

Surprisingly, there is actually little to none disk reads, and only about 1MB/sec writes happening (but those constantly). I see about 43IOPS in the resource monitor by Synology.

So, what might cause this performance hit?
Is 1.000 files in 10min a common number?
Any ideas of what might cause the high disk IO?

Many thanks in advance! Any hint or tip is welcome.