Docker Ubuntu Dashboard Classic Graph issue

Steps to reproduce:

1. Open Dashboard that includes a classic Graph
2. Dashboard opens, shows all Widgeds, Graph not shown or shown after a long time

Result:

top shows a lot of php-fpm8.1 processes with 100% CPU 
System-Load increases fast (some times >50)

Test:
extend to 16 CPU Cores/16GB Ram, 2 Users, 2 Dashboard, 5 Graphs 

Result:
16 CPU Cores at 100% >15min, system load ~20, some processes with 2G RAM reserved

Workaround:

using zabbix/zabbix-web-nginx-pgsql:6.0-alpine-latest 

graph is displayed immediately, no CPU load

i suspect that it might be related to the php version. Ubuntu uses PHP8.1, Alpine PHP8