Persecution usually slows down growth - as shown by increased growth after the persecution stops, as in the past in Spain,
Not necessarily metraton. My parents were missionaries/pioneers in Spain at the end of the Franco era when the witnesses were being persecuted still. The numbers didnt go up because they were no longer banned as such - just the numbers could be assessed by attendance at the halls etc. During the time of persecution my parents experienced quite a lot of success bringing people into the org, only it wasn't as easy to baptise them whilst everything was underground.