Slow parameter transfer via USB since 2.0.11

After upgrading from 2.0.10 to 2.0.11 the parameter transfer via USB connection is painfully slow. With 2.0.10, the transfer of 414 parameters took less than a second, now is about 5-10 seconds (varying). The new behaviour is somewhat similar to the transfer via 3DR radio now: fast transfer of the first 100 parms, than very sluggish transfer of the rest.

Used OS: Fedora 19/x86_64, compiled from sources.
FC: Pixhawk, current master

No files, tests, or entry points are named. Reproduce parameter transfer with a Pixhawk over USB, comparing 2.0.10 and 2.0.11 on the reported setup, then trace the parameter-transfer path and review the changes between those versions; done means restoring sub-second transfer for 414 parameters without regressing radio transfers.

Written by the indexing model from the issue text.