same field in bothe "where" and "order by" - how to index for this query?
The OR will most likely affect the index usage, because it hinders index seeking. Generally speaking "OR" is 'bad' for index usage in where clauses.

The time field, it would be sensible to have it in an index sorted in the DESC order when you use it DESC;

Other than that - we'll need more information to give more exact/concrete advice