I did some research this week because I have the same issue with the Activity Log. What I found is very simple, you need to wait. When you use the Activity Log for the fierst time, it collect so much data, that's why it takes so much time. I did the test yesterday, just put the Activity Log on SysNAND and wait. After 1 hour, the app finally open. I close it and open it again (just to check it) and it works inmediatly.
The problem I have now is that, after SysNAND I went to EmuNAND and did the same. wait. At this time, my 3DS has 22 hours consecutive trying to open the Activity Log. i think I'll just wait until it opens.