Calendar entries are small. I have 191 that take 0.27M. Considering that I have 18M free, I could have 5 000 calendar entries without running low on memory. That would be a lot, even for a wedding planner...
I use my calendar for everything...usually an average of 8 enteries everyday. I haven't had any issues with speed or performance.
I did read that 1 person suggested that under Options (in Calendar) that you avoid the Forever selection. I have mine set at 90 days.