PLINQ executes work items in parallel and, by default, doesn't care about preserving the order of items to improve the performance of parallel queries. However, it is sometimes important that items are executed in the same order as they exist in the source collection. For example, imagine you are sending multiple requests to the server to download files in chunks and later on merging those chunks to recreate the file on the client side. Since the file is downloaded in parts, every part needs to be downloaded and merged in the correct order. Preserving the order while executing items in parallel has a direct impact on performance as we need to preserve the original ordering throughout the partitions and ensure that the ordering is consistent when merging items.
We can override the default behavior and turn on order...