GP Performance: Number of map layers in mxd, Foreground vs. Background processing

Please help me understand how background/foreground processing performance relates to the number of map layers in the open ArcMap Map Document. Here is an observation that I'm trying to make sense of:

The number of map layers in the map affects the processing time of (out-of-the-box as well as Python Script tools) gp tools when they run in the foreground. Having 100s of layers in the .mxd's TOC makes gp tools EXTREMELY slow (i.e.1hr instead of 1 minute), independent of whether these gp tools actually interact with any of the map layers or not. When gp tools are executed as background processes they are not affected.

Interestingly, adding the same number of map- or feature service layers does not have an impact at all. For example, I can add 500 floor plan layers as a map service and still get good performace on foreground gp tools, but when I add them as map layers based on feature classes the performance of the gp tools is unbearably slow.

Database is Oracle 11g, ArcGIS is version 10.1 SP1.

I also posted this in the GP forum - not sure where the appropriate place for this is. let me know and I'll delete one or the other.

Thanks!
Tobias