Helen- We've all been there, so here are few things to consider regarding workbook performance,

1. Do you need to load so much data? - Consider data source filters, context filters to reduce data size.
2. How about creating a data extract? - Extracts improve workbook performance significantly.
3. Avoid complex calculations on the fly - If possible perform aggregations, calculations at database level.
4. To your 2nd question, Tableau queries database server and depending on the complexity of the query the CPU is used. If you're loading such huge amounts of data and calculations into view the query gets complex.

Here are some useful links,

Improving Dashboard Performance

Performance Checklist

Optimize Workbook Performance

Record and Analyze Workbook Performance

Improve Workbook Performance

Hope this helps. Good luck!

Thanks,

Shiva.