If you really want to reboot the VM, you’ll have to do it through UCS or your hypervisor (ESXi).
What appears slow and sluggish? The WebUI? ownCloud WebUI or UCS appliance WebUI? Can you still login via ssh?
Where exactly and how do you do it? Because on a UCS appliance there could be problems with the ownCloud docker container, other docker containers that are part of the stack, the database, the docker service, any other part of the system.
Typically it can be pretty hard to debug performance problems if docker is involved as there are so many different stacks.
Have you considered memory issues (not enough for example), storage throughput issues and network throughput issues? Monitor these metrics and have a look if there coincides a spike during this time and I’m pretty sure you’ll find the culprit.
Performance issues can be very hard to debug, I wish you the best of luck!
personally the first thing it would do is indeed to increase the memory. 1.5 GB looks to me quite low when reading that the UCS appliance has a quite complex setup:
As an alternative maybe https://help.univention.com/c/ucs could be another support possibility as the appliance is based on UCS where deeper knowledge about the UCS is required for such kind of issues and where i’m not sure other users in this forums have them.