I agree, although incrementing and decrementing are trivial enough cases that handling them on the Nextion side would not be at all unreasonable.You'll be better off in the long run to treat the Nextion like any other display that you would use. Use it for a display and not for a processor.
Of course, IMHO and YMMV.
I find that the part of my code handling interaction with the Nextion is processed a number of times a second--such that delays from the back and forth to the picaxe are not noticeable to the user. For other circumstances, that would be one of the areas where YMMV, and offloading processing to the Nextion could improve response. I have not thus far found that to be needed.