Hi,
one thing that I found out that improves performance is an index in DESCENDING order on the timestamp field. At least on MongoDB 6.0.0 this has different behavior. A compound index on meta field + timestamp field does not provide the same performance. It will still load and unpack all the documents that match the query criteria and then sort and limit.
You might try to enforce the usage of the index by using the .hint() function. https://www.mongodb.com/docs/manual/reference/method/cursor.hint/
Cheers,
Martin