I'm using Stimulsoft.Net version 2015.2.1 and have a huge difference in rendering time between ConvertNulls on or off.
Just add a DataBand to the report and place a field which contains null values to this databand.
Rendering with ConvertNulls = true: 1 second.
Rendering with ConvertNulls = false: 120 seconds

Can you check this please?
Thanks & Regards,
Matthias