It depends.
The number of attributes doesn't really affect speed... unless they're all large blobs- in which case they would fill the memory buffer and you'd feel the speed decrease. But it's not the number of attributes- it's the size of the data set.
Generally, fast queries come from an efficient indicing scheme. Make sure you have indices only on attributes you need to index- indices will speed up selects and slow insert, update and delete.
Do your complex queries require multiple joins? Joins will slow things down without efficient indexes. It doesn't mean you can't do them... just make sure your tables are normalized somewhat, with efficient indexes, and that you eliminate duplicate information.
