The answer
Yes. Every index adds overhead to writes because MongoDB has to update each index. Indexes are a tradeoff: faster reads, slower writes. Do not index every field; index the queries you actually run.
Compound Indexes in MongoDB Explained
Do compound indexes slow down writes?
More FAQs in Compound Indexes in MongoDB Explained
An index on two or more fields. MongoDB can use it for queries that filter on any prefix of the indexed fields. Example: index on (fromUserId, toUserId) works for fromUserId alone or fromUserId + toUserId, but not toUserId alone.
A compound index on (A, B, C) can be used for A, A+B, or A+B+C. It cannot be used for B, C, or B+C. If you query on B+C, you need a separate index on (B, C) or a different compound index that starts with B.
By the ESR rule: Equality (exact match) first, Sort second, Range last. For a query like { fromUserId: X, age: { $gte: 18 } }.sort({ createdAt: -1 }), the index would be { fromUserId: 1, createdAt: -1, age: 1 }.
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