Fauna is a temporal database, not a time series database. The code in the test that updates the score on every post after every read is creating new historical events every time it does that. These have to be read and skipped over during the read query which will continually increase latency proportional to the number of updates that have occurred.
By default, Fauna retains this data and makes it queryable (with transactional guarantees) for 30 days, unlike DynamoDB or Redis. Reducing the retention period would help a bit, but event garbage collection is not immediate so there will still be differences for heavily churned documents. Normally, having a few updates to a document or an index has no noticeable impact but in this case it appears to be swamping the other factors in the latency profile.
It is possible to manually remove the previous events in the update query; doing that should reduce the latency. Nevertheless, Fauna is not a time series database so this is a bit of an anti-pattern.