Why is this stock iMac so slow at launching apps?
It could very well just be the slow-ish hard drive. Apple generally does not put 7200RPM drives in its Macs. While it could be the O/S and it might be worth your while to image the internal drive onto an external drive and wipe the internal and start fresh with the OS and Apps. Depends on what is more valuable to you time or money.

An SSD WILL speed up your system, often it's like a whole new Mac, depending on your usage. A deep cleaning of the current OS or a wipe/reinstall might fix the issue or at least improve it.