Definitely interesting how performance between different cloud DB services is so untransparent, similar to object storage [1]. Well, it wouldn't matter much with CSP lockin.
In addition to factors you can't control, maybe Digital Ocean has different Postgres params? I know MySQL for RDS has a bunch of knobs you should tune [2].
[1] https://dev.to/sachinkagarwal/public-cloud-object-store-perf... [2] https://aws.amazon.com/blogs/database/best-practices-for-con...