The first thing I'd do is to turn on the slow query log (or lower the threshold). Once you have a few logged, run an explain/analyze on them and see if it's an issue with the queries.
On the instance itself, you can check memory usage (free) and cpu usage (top).
Most of the popular graphical clients for PG (I've used pgadmin a little) have some way to show current resource usage as well. PG keeps quite a lot of statistics by default, the manual is quite good: https://www.postgresql.org/docs/current/performance-tips.htm...
Check the EC2 dashboard for your instance as well, to see if any other resource usage is high.