does number of fields in a table affect query performance

[Sava Rados]

Hello all,

I have big table (InnoDB): 70 fields, 6 million records. I have another table with 10 fields (fields that I currently use for queries) taken from first table and same 6 million records.

Identical query (forcing identical indexes) over the table with fewer fields works some 4 times faster.

I wonder how rest of fields can affect the query when they are not even used?!?

Thanks