I cannot confirm running on less than 5s and iPad 3...but my app does have an x-axis that is a date axis...I'm about to release a new version of code with OxyPlot in it. Thus if there is a major issue with the 4 I'm concerned.
Basically with Instruments I'm looking for where all the time is being spent. From your snapshot start at the top and keep hitting down arrows until the function that is using up all of the time is exposed.
Although I can't see exactly where the code is falling down, I can at least point you in the correct direction. From your Instruments after I clicked down through all of the calls:
TimeSpan axis correct? OxyPlot/Axes/TimeSpanAxis.cs line 173
protected override double CalculateActualInterval(double availableSize, double maxIntervalSize)
From Instruments it looks like the line 188 is just spinning
double nextInterval = goodIntervals.FirstOrDefault(i => i > interval);
I'd start poking around here with the debugger and see why that
is never ending, or why it is being called so many times