Why there is performance degradation after ~6 hours of Java 9 G1 work without the actual increase in load?
(Note that GC tuning is extremely dependant on the environment, so there is no magic recipe.)
Had a very similar issue with G1. By default, it seems to be rather badly suited for REST endpoints (again, this is only what I experienced in my direct neighborhood). What helped me was experimenting with the GC flags, as described here.
For us, the biggest improvements came from -XX:G1NewSizePercent=25 and -XX:MaxGCPauseMillis=50. G1 is also auto-tuning itself over time, so the max. GC pause limit has a significant effect on all other parameters.