It does so in other hosts (i.e. Live) too. Please report this to the plugin devs. If there’s something we can do to prevent this, they should/will contact us.
but this happens NOT in renoise 2.1 , NOT in 2.5, it started to behave like this since 2.6.
so you may know what new routine in renoise, invented in 2.6 cause this ?
edit:
tested in wavelab6 and vegas7: no cpu load with that plugin opened.