Hello Florian,
I had the same problem with my first zentyal installation. Openchange got over the course of a week slower and slower until it immediately crashed when there was some load on (With load I mean one web interface instance running).
Apparently, after reinstalling, openchange is faster, however it is still very slow (the webinterface takes sometimes over 2 minutes to completly load (in lan)) and crashes sometimes (~2 times a month). The solution for the crashing was a cronjob which sends a start command to the zentyal-openchange service hourly.
Outlook (2016) still also shows the server down a lot however not all the time. Receiving mails in outlook however even works when the server is shown down. I came around this by using imap instead. Strangely, on a virtual machine (2 cores of my Intel i5-3317U mobile, 1,5GB DDR3) I sat up for testing before changing something on my production server, I don`t have any speed issues. I´m aware that the server has to handle a lot more tasks (dhcp, dns, users, samba filesharing, cups, webmin) than the virtual machine (only openchange and users). However, I´m running less than 10 users (it´s a private server) on a Intel Xeon E3-1230 with 8GB DDR3 and a 250GB Samsung SSD as the system drive so speed issues with this configuration should not happen. Also the CPU load is below 1.0 and I´m barely using a quarter of the available ram.
I haven´t tried any other machines but maybe openchange has problems with different types of cpus?
greetings,
Reisperbachtal