Very slow simple PostgreSQL query on RDS
Matching index
After re-reading your question I realized you are not running Amazon Redshift, but Amazon RDS, which seems to be running unsullied Postgres, at least according to the documentation:

Amazon RDS supports DB instances running several versions of
PostgreSQL. Currently we support PostgreSQL versions 9.3.1, 9.3.2, and 9.3.3.

This would mean you have index-only scans at your disposal. If you meet some preconditions (basically if vacuum can keep up with write operations) and if some_field is not updated to often and reasonably small (which seems to be the case for a numeric column), the perfect index would include some_field in last position (like @zerkms first mentioned):
CREATE INDEX ON pages(some_id, first_action_at, some_field);

Note that some_id should come before first_action_at, because it is typically more efficient to have columns with equality checks first and ranges later. Details:
Multicolumn index and performance
If you don't see &quot;index-only scan&quot; in EXPLAIN ANALYZE, the last column is just ballast and better left away:
CREATE INDEX ON pages(some_id, first_action_at);

(Like you have now, according to your question update.)
Either way, another index on just (some_id) only offers very little over this multicolumn index:
Is a composite index also good for queries on the first field?
Server configuration
All the usual advice for slow queries and proper server configuration applies and a work_mem setting of 1 MB is much too low for a DB with millions of rows. But this particular setting should not be crucial for this particular query, since work_mem is (per documentation):

memory to be used by internal sort operations and hash tables.

Neither applies here.