I know this thread is older, but I figured I would chime in because I have been running into problems with using in_memory workspaces. When it comes to using the in_memory workspace in the interactive python window when background processing is enabled; well, all bets are off. There are numerous tools that don't actually create GPInMemoryWorkspaces when using 'in_memory' with 'Background Processing' in the interactive Python window. I primarily work in ArcGIS 10.1, but I have also noticed the same is true with 10.2.1. If 'in_memory' doesn't mean in RAM, then performance could be much lower than expected. If using 'Background Processing' with the interactive Python windows, it pays to double check whether your 'in_memory' objects are actually in memory and not in disk-based temporary file geodatabases.
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