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Updating performance-best-practices.md (#22192) 

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Co-authored-by: Rick Anderson <3605364+Rick-Anderson@users.noreply.github.com>

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Co-authored-by: Luke Latham <1622880+guardrex@users.noreply.github.com>

Co-authored-by: Luke Latham <1622880+guardrex@users.noreply.github.com>
Co-authored-by: Rick Anderson <3605364+Rick-Anderson@users.noreply.github.com>
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@ -42,7 +42,22 @@ A common performance problem in ASP.NET Core apps is blocking calls that could b
A profiler, such as [PerfView](https://github.com/Microsoft/perfview), can be used to find threads frequently added to the [Thread Pool](/windows/desktop/procthread/thread-pools). The `Microsoft-Windows-DotNETRuntime/ThreadPoolWorkerThread/Start` event indicates a thread added to the thread pool. <!-- For more information, see [async guidance docs](TBD-Link_To_Davifowl_Doc) -->
## Return IEnumerable\<T> or IAsyncEnumerable\<T>
## Return large collections across multiple smaller pages
A webpage shouldn't load large amounts of data all at once. When returning a collection of objects, consider whether it could lead to performance issues. Determine if the design could produce the following poor outcomes:
* <xref:System.OutOfMemoryException> or high memory consumption
* Thread pool starvation (see the following remarks on <xref:System.Collections.Generic.IAsyncEnumerable%601>)
* Slow response times
* Frequent garbage collection
**Do** add pagination to mitigate the preceding scenarios. Using page size and page index parameters, developers should favor the design of returning a partial result. When an exhaustive result is required, pagination should be used to asynchronously populate batches of results to avoid locking server resources.
For more information on paging and limiting the number of returned records, see:
* [Performance considerations](xref:data/ef-rp/intro#performance-considerations)
* [Add paging to an ASP.NET Core app](xref:data/ef-rp/sort-filter-page#add-paging)
### Return `IEnumerable<T>` or `IAsyncEnumerable<T>`
Returning `IEnumerable<T>` from an action results in synchronous collection iteration by the serializer. The result is the blocking of calls and a potential for thread pool starvation. To avoid synchronous enumeration, use `ToListAsync` before returning the enumerable.