If there are only a few people recordings are fine but when there are a lot of people in the picture moving around, the recording can studder and in some cases become pixelated. Is it the camera or Blue Iris? They are all Hikvision or Dahua and all seems to have the same issue.
Is this on one camera or all cameras ?
Are you running headless ?
What is the wdr setting on the camera ?
What is the make and model number of you cameras ?
Is the problem both day and night ?