Most probably, your QVS service has been eating RAM like candy, up to the Max Working Set Limit. QDS (the distribution service) does the reloads as a set of processes outside of QVS. As a result, QDS may find that most of the machines RAM has been allocated to to QVS and because of too few memory bytes available for doing in-memory reloads will start paging to disk. That's the cause ot the slowdown.
You can either:
- buy more RAM and tune the Working Set limits so that QVS doesn't run off with all of precious memory
- restart the QVS service before the big reloads are launched (for example at night)