slow category structure

[CoolCarPartsOnline]

It all depends on the server and how you have it setup, the database size come into play when you have a lot of queries.

[DrByte]

With a large database would I be better to only put long descriptions in the category and not each product? or both?

It all depends on the server and how you have it setup, the database size come into play when you have a lot of queries.

Even more significantly it depends on whether that approach suits your business and your products.
I would suggest that having large product descriptions is not going to significantly slow down the database, since all the "performance" stuff has little or nothing to do with the data in the table where your product descriptions are stored.
So, in short, do what's best for your business. Either will be essentially equal impact on the database operation.

The only effect larger descriptions might have is making your backups larger. But disk space is cheap.