Not sure what you mean by 'query with WHERE-clause or without', but you're correct that most of the time a query with a WHERE clause on an indexed field with outperform a query whose WHERE clause on a non-indexed field. 

One instance where the performance will be the same (ie indexing doesn't matter) is when you run a range based query in your where clause (ie WHERE col1 > x ). This forces a scan of the table, and thus will be the same speed as a range query on a non indexed column.

Really, it depends on the columns you reference in the where clause, the types of data in the columns, the types of queries your running, etc. 

Performance of SQL query with condition vs. without where clause
We've found that our users have almost universally benefited from actually going through our walkthrough. Obviously, we have some more ux work to do on our service. But, based on this information, we decided to force users through the short walkthrough in the interim. Somewhat unsurprisingly, people have protested a bit :) So, what do you think, is it ok to force users through a brief walkthrough when you know it will benefit them in the end?

Edit
We are going to add some mixpanel hooks to find out if users are leaving when presented with a walkthrough they can't exit out of.
