What is your update rate for that channel? The maximum number of points you can read in a single plot is 8000. Except in limited cases, you can't get more by averaging in the ThingSpeak charts. You can use the charts API to specify a daterange for your chart, or create a custom MATLAB Visualization to show a different collection of points. This example shows one way to read past dates.
You can also select a web site from the following list:
How to Get Best Site Performance
Select the China site (in Chinese or English) for best site performance. Other MathWorks country sites are not optimized for visits from your location.