There aren't any issues that I know of. Office 2013 didn't introduce any new data formats for excel. It's still the XLSX format that's been the same since office 2007.
I'd imagine your performance loss is the same thing I experience with every new version of office, that it becomes even more of a resource hog and slows down systems that are >2 years old significantly. Try comparing the system resources excel in using in the old vs. the new one to confirm.