Are nulls slow?

I have several tables anywhere from 50,000 to 100,000 rows with about 100 columns in each that allow nulls. How bad will this affect performance (if any)?

If it's significant, how can I replace nulls with zeros without doing it column by column? (100 columns will take too much time)

thanksI dont think null will affect the perfomance.Below code will generate update statement for ur tables,columns which allow null values.Run this code, copy and paste the result to query analyser and execute.

select 'update '+TABLE_NAME+' set '+COLUMN_NAME+'=0 where '+COLUMN_NAME+' is null'
from INFORMATION_SCHEMA.COLUMNS
where IS_NULLABLE='YES'