Transfer speed of Drobo vs. internal drives?

If you're connecting over gigabit ethenet and you don't have super-fast HDDs, LAN will not be the bottleneck. USB, however, could be a serious bottleneck. Rough estimation of speeds:

Item                 Bytes/s      Bits/s
----                 -------    --------
USB 2.0:              60MB/s     480Mb/s
7200RPM HDD:          70MB/s     560Mb/s    (average hard-drive)
10000RPM HDD:        100MB/s     800Mb/s
Gigabit Ethernet:    125MB/s    1000Mb/s
2x 7200RPM RAID0:    300MB/s    2400Mb/s
FireWire 800:        400MB/s    3200Mb/s

1 Byte = 8 Bits

You would definitely get lower speeds over USB, with a Drobo or anything else. All the drive's I've attached via a USB2 port top out at a little over 25Mbyte/sec for bulk operations - much slower than your internal drive controller will manage and with a CPU hit due to the design of the USB protocols too.

If your LAN is gigabit then you should see better rates than USB for bulk operations - about 50Mbyte/sec is the best I've seen between machines over gbit ethernet, though I've not tested this extensively or at all scientifically. This will still be slower than internal drives under ideal circumstances (bulk reading with few head moves) though for random access the difference will be less (once the drive heads start needing to move around a lot their latency will become the bottleneck not the throughput bandwidth available between the controller and the PC.