I have been working on large ecommerce sites for the past year or so. Large as in 25,000 products or more. The majority of the sites use a tricked out version of magento, but as with any CMS, on-page optimization can be a bitch. Also, optimizing 25,000 pages is just not practical.
I have a team in India that we use for development and also for repetitive tasks. In order to have them work on a project, the steps need to be documented in a simple format that anyone can just pick up and follow. So I have them do what I call the 4 step optimization.
The majority of the site content is dictated by the product feed that is uploaded into the site, so things like Meta info, short and long descriptions, etc are decided in a column of a spreadsheet.
When you have too many pages to optimize or the benefit is not big enough to justify the time, create a system to:
1. Put the keyword first in the title tag
2. Put 3 or 4 keywords after it that are either an application that is related to the keyword, or the category the keyword falls into. (this rule works for most products)
3. H1 Tag with the keyword
4. Body text with keyword
5. A link or two with keyword in the anchor text
I know this is no ideal, but on-site SEO is still extremely important.
We just launched a 35,000 page website, new domain, no links, all of the organic traffic is coming from the onsite SEO and recently, shopping feeds. Even though I have been doing this since 1996, this situation really drove home that on-site SEO cannot be ignored. Systemized maybe, but not ignored.
I believe that a page can be optimized to be "good enough" in under 2 minutes.
Hope you're all doing well.