Yes it could. You don't say what database you're using or what data type the extra columns are, but adding more columns is going to increase the 'width' of your table which means that more logical reads are needed to retrieve the same number of records, more reads equals slower speed. So what you gain by denormalisation may eventually be lost by adding too many columns, but the extent of this will depend on your database design.
If it does affect performance an intermediate solution could be to vertically split the table placing infrequently referenced columns in a second table.