Adding more RAM to the system can significantly improve performance, especially in scenarios where multiple applications are being used, as it allows the system to handle more data at once without relying on the slower hard drive for virtual memory. Uninstalling unused applications might free some resources but is unlikely to have a significant impact on performance when multiple applications are running. Fragmented data could slow down hard drive access times, but defragmentation is typically not the primary solution for performance issues related to running multiple applications, and it wouldn't be beneficial for SSDs.