I have an old Project Management spreadsheet with at least two dozen pages, that presents no problem with Excel RT. When you say full, how many rows and columns are used and how many are formulas and how many are constants.
Nope no major lagging with mine on my surface. May I suggest that you make sure that your OS is up to date. Also you may want to make sure that you don't have a lot of apps open (due to the lack of ram in the surface RT)
A common misconception is you have to close the Modern UI applications, which is not needed. An application not visible will be suspended after 5 seconds, and will not be scheduled CPU cycles, unless it is configured as a background task. A suspended application is a candidate for termination, i.e., removal from memory when another visible application requires memory allocation.