Okay I have this on my device now. It’s an iPhone 5S. It’s reasonably fast. It takes about 1 second to transition back from the detail view to the calendar.
I’ve also got it running on a very old iPhone 4S. It is slow. About 2 to 3 seconds.
I’m not sure this can really be optimized much more. It’s got 3 slides - each with pretty intensive layouts.
I did remove the last 2 slides and performance on the 5S is perfect. Performance on the 4S is pretty good - less than 1 second transition.
I feel like you might need to refactor to not use a Slide Box for this. How about just using a single view and using a swipe detection to change the calendar out? Possibly use little arrow indicators on the left and right about middle of the page to indicate previous month and next month? You could also use buttons in the header to show that.