This is driving me crazy. Has anyone got a list of the plugins that suffer from this? Perhaps we should start one.

A brief overview on the topic, from Digital Fish Phones (Dead Link Removed)

Denormalisation is an issue with some audio hosts that don't switch off the plugin 's audio task when the song is stopped or a track contains silence (or with instruments: no key is pressed while a sound decays).

The problem happens when the CPU (precisely: the floating point unit (FPU) inside of it) detects extremely small numbers and wishes to process them with the same precision as usual. Then it switches into 'denormal' mode which is another way of representing small floating point numbers with the available bit range.
Converting between the two states takes a lot of time for the FPU and thus might cause heavy load jumps.