Any insights from the APM? 

what's your API Builder version? I mean, if you haven't uploaded a new API Builder plugin, then it would not have any changes. Try checking your plugin version, or maybe there's a new update in comparison to yours?

Have you tried using 3rd party tool like Postman? if its also slow on postman, could be something wrong with the API service itself. Check if there's any service outages or slowness on their end. Also, sometimes API services might implement rate limiting that could slow down reequests if you exceed the limit. probably have to check if there's any rate limits on the API service.

Else, it could be an infra thing:

• any network-related issues where latency or packet loss could increase the response time
• any firewall or proxy? worth checking there as well.
• How's the CPU or memory usage on the server? if not enough resources, could impact the response time
• if your API involves database callls, check if there's any bottlenecks or anything