While investigating why webdav protocol was slow on my Nextcloud server, I put up a blog post explaining how to profile Nextcloud.
What’s great about profiling instead of relying on the logs is you can investigate any slow down without prior instrumentation from the devs.
Profiling slows down execution though, so we’ll see how to enable it on demand only.
See for yourself at ibizaman's Blog - What's up with Nextcloud webdav slowness?.
TBH, I have no idea what my router is actually doing
NAT Loopback or Hairpin NAT / NAT Reflection are the terms you are looking for: Network address translation - Wikipedia
but I circumvented by adding
nextcloud.domain.comto my internal dns server.
Which is a better way to do it anyways. This concept is called Split DNS or Split-horizon DNS. 
@bb77 awesome, thank you! Those were the keywords I was missing!
I updated the post with your explanation
thanks for sharing!