I think the hardcoded limit in Admin is 250. However, I don't think 250 active rows in a single custom section on a specification would perform very well. I would try not to exceed 30 active rows per section per specification. Performance involves so many variables, it will depend on the type of extended attributes you have used in each cell. For example, if you had one section with 10 columns and 30 rows and every cell had 15 radio button choices the section would take quite a while to render. If helpful, you can always submit your custom section designs to support and we can review it for any potential gotchas.