thanks to the rapid response; I was hoping to avoid the upgrade for the moment, but I’ll try to install a parallel demo environment; mysql or postgresql?
if Nr. 3 and 4. doesn’t help that much then i think there is no way around upgrading to a recent oC version + PHP7. There is a blogpost about performance improvements in PHP7 available at [1] which shows you a few impressions. IIRC the upcoming ownCloud 10 will bring even more performance improvements due to code optimizations.
Besides that the old 8.1.0 should be really upgraded to at least 8.1.12 [2] as this version contains various public known security vulnerabilities as shown in [3] (not even talking about the tons of known bugs )