I am unable to reproduce your error. I have a case with 4 hold and I get a decent response time, see the screen shot below.
I have noticed that you are using beta endpoint. You should try the v1.0 endpoint if there is no specific feature in beta that you are looking for and consider the following.
- Run a one off request through v1.0 on postman or graph explorer and share the result.
- Describe how persistent the issue is on your end.
- Was this issue happening before or started recently?
- How many legal holds does the case have?
If you still face the issue on v1.0 and it happens persistently, share the result and you will need to create a support request from compliance portal.