The increase in speed is more liekly due to the fact that Lr constantly reads and writes data to and from the catalog.
When you do a further step in the Develop Module, this step has to be saved in the catalog and the whole lot of data representing all develop steps regarding this photo has to be read from catalog.
This is why with Lr hard drives can become a real bottle neck, particularly in systems where the OS, LR, the catalog, the Lr cache, and photos are on the same disk.
And an SSD will bring a performance boost because the reading and writing is so much faster.
Apart from that, there is no way to force Lr not to use virtual memory. But you can help Lr by storing the various files (catalog, cache, and photos) on different physical drives (partitions won't help) so that the reading and writing will be done on several drives simultaneously.
My catalogue is on a separate disk already - so that cant explain the speed increase :/