It's unforgivable that MS release Excel with a lot of flaw in performance (it's a calculation program for a God sake).
The fact that Excel uses strong key generation to secure your document when you click the "I would like to secure my Excel document with strong key generation" is not a performance issue. It's by design. In fact, if it were fast to open large numbers of password protected files, it would be fast to brute force passwords, and someone would log it as an Important bug at Microsoft and tell them to fix it.
If you're talking about some other performance defect in Excel, that's fine, but open a new thread to discuss it. This particular thread discusses slowness in opening password protected documents. This slow down is by designin order to keep your documents safe, and if you don't like it you can turn password protection off.