When using the context delete approach as shown, we are loading the data of the object first and are then generating the respective delete statements for the server.
Depending on the amount of data this could take some time. In order to dig deeper here, you can enable logging to see which operations are performed against your database.
In order to improve the deletion of objects I would like to recommend the bulk delete API as described here. By using this approach, you are no longer loading the objects first but the delete operation are pushed directly to the server.
Hope this helps.
Do come back in case you have any other question.
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