andi However before we investigate further into that direction, there are a couple more question I can ask:
can your server (and thus DokuWiki) reach the Internet?
Yes; I can update dokuwiki, install plugins, etc. as intended.
is the update check enabled? (if yes try disabling it)
No, it is disabled.
is the dnslookup option enabled? (if yes try disabling it)
It was switched off. I enabled it now for to feel some changes.
has it always been this slow? (if not, what changed?)
I feel, that it is perhaps related to bootstrap-template, but I am absolutely not sure. Just in this moment I access the wiki from South Brazil and it seems quite okay. Frankly speaking, I cannot judge whether it was always this slow.
you mention you run a carddav server on the same system - is access to that slow too?
This carddav is somehow integrated into the Mac OS-server. We only use is for sharing addressbook; I cannot complain concerning "speed".
is there any significant server load (CPU/Memory usage spikes) notable when an access comes in?
I will check the next time; for the time being I would say "No" because I assume I would have noticed that somehow. Correction: I just looked at the server via Apple screen sharing and sometimes process httpd (no idea what it is) shows CPU 100%.
you say you experience the problem when accessing via port forward from extern only. Does this mean accessing the wiki from inside the network is faster?
It is difficult for me to answer this correctly. But I would assume the access via the port forwarding is really slower.
PS: PHP 5.6 itself is slow compared to modern PHP releases and will not be supported in future DokuWiki versions anymore. But it's for sure not the reason for a 15 second delay, so let's ignore that for now.
I have already a spare machine running with php 7.x; do not know exact version. I currently do not want to switch over to this one because up to now I did not get carddav running on this. But I will check, whether access to this one is faster.
Perhaps I made a mistake concerning the port forwarding: I just forwarded port 31xxx to the macbook-server and with: https://82.345.258.76:31xxx/company_wiki/doku.php I access the wiki. (The IP is a fake-IP)
What I can do next time when I am back in the office is to install a fresh dokuwiki on the same machine and check whether this works much faster.