Lets say you have an object in salesforce with 300 fields.  I then create two records, one where I only fill out 2 of the fields, and another record where I fill out all 100 of the fields.  My questions are: 1) Do both records take up the same amount of space in the database?  2) What determines the speed of a database on salesforce; the total size of how much space the database file takes up in MB?  Or is it based on the number of actual records (big or small) that are stored in the database file?

I have a salesforce org I'm working on with almost 300 fields and half a million records and it chugs like crazy (constantly times out when running queries and cant even open the tab view without timing out).  I think part of the problem is that the initial designer (a sales rep at the company) put every field he felt they could ever possibly need on the object (that only a couple records might ever use) instead of creating a seperate object to attach all these little bits and pieces of random information to.  As a result, there's always a ton of empty fields in the various records and I'm wondering if that might be a part of the problem.

Either way, any ideas or suggestions about working with objects containing millions of records would be hugely helpful.  We've already asked salesforce to add custom indexes to fields that get queried a lot, so I'm not sure what else can be done to speed things up.

Thanks.