Large delay in export processing a VST (MIR Pro Vienna)

MIR Pro is Vienna’s reverberation application. I tried to use it as a VST effect in WaveLab Elements 9.
MIR reverberates what it promises, however, it takes 40 seconds to start in an export: The first approx. 40 seconds, the output is unprocessed and after this processing starts.
This makes any export unusable.
When fiddling with the effect in the montage panel, such a delay is not experienced.
Is this a WaveLab bug or something else?
Jan Herman