Kibana slow performance optimization
So basically I've ended up turning these flags off (/app/management/kibana/settings)
filterEditor:suggestValues
discover:searchOnPageLoad
doc_table:highlight
visualize:enableLabs
telemetry:enabled

Other ideas I had in mind:

adding nginx as a proxy caching all the static files (js for plugins / images)
warmup script (periodically makes requests to predefined hot pages for Kibana to internally cache these pages) if that would work
migrate to hosted cloud solution which might be optimized better for the loading time