Help Optimize Query: Finding matching Answer in a log of Queries and Answers

Hi,
Being very new and naive to Power Query, I've managed to create a query that works, but that ends up being unacceptably slow.   I'm sure there is a much better way, and I would appreciate your guidance !

Context: my source table contains a log of events, where the main columns are:

1. [QueryID] - a string than identifies a transaction/query
2. [Status] - The type of log entry associated with the Query, one of "Opened" (=request issued) or "Answered" or "Closed" (= responses to the request ... "Closed" is for now equivalent to "Answered").
3. [Changed At] - a timestamp

Small complication: within a single [QueryID] there can be more than one request "Opened" and then "Answered".

Goal: I want to extract a list of recorded transactions (opened & answered), along with the time it took for the transaction to be answered.

Step-by-step, I have manged to write the following query which delivers the correct result, but gets inacceptably slow when confronted with large data sets:

= Table.AddColumn(#"Sorted Rows", "End At", each if [Status] <> "Opened" then "--" else
let qq=[QueryID], minT=[Changed At]
in List.Min( Table.Column( Table.SelectRows(#"Sorted Rows", each ( [QueryID]=qq and ([Status]="Answered" or [Status]="Closed") and [Changed At]>=minT ) ), "Changed At") ) )

For all "Opened" message entries, this will find a corresponding "Answered" entry (the closest subsequent entry in chronological order), retrieve its timestamp, and store it in a new colum "End At".  This way I will get a list of all the intervals.

Of course this gets really slow for large logs.  I can think of ways to optimize this by creating separate intermediate tables and queries.  But I strongly suspect there is a much more straightforward way to perform the above operation, which may avoid manual optimization...
Can somebody help, and teach me proper tricks ?