## The Problem
When grouping messages, the code was accumulating recordsets using the `|=` union operator inside a loop. Since each union call internally builds an `OrderedSet` over all previously accumulated IDs, the performance degraded quadratically relative to the number of document records. This caused bottlenecks on databases with large message volumes.

## The Solution
* Replaced the `|=` recordset accumulation with a plain Python dictionary of ordered sets to store IDs per operation, while keeping same behavior.
* Deferred the `browse()` call until after the loop is complete.
* Reduced the overall complexity from **$O(N^2)$** to **$O(N)$**.

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## Benchmarks
*Tested on a customer database grouping by "Created By" and "Created On":*

| Record Count | Before | After | Improvement |
| :--- | :--- | :--- | :--- |
| **300k records** | 83.00s | **1.00s** | **-99%** |
| **30k records** | 0.60s | 0.25s | (Minor) |

**Note:** The performance gains become exponentially more significant as the record count grows.

**OPW-6123758**