I think it is not specific to the autodialer but comments have been made that the current implementation does not scale well^1 with many changes to the road matrix.
Probably the current implementation (beebrain) is such that it walks the entire datapoint and roadpoint (road matrix) lists validating datapoints, calculating rates, values, dot colors, and so on and currently must do so from the beginning til ‘now’ to create/calculate the graph (and stats).
Thus my curiosity about cropping the displayed graph range (tmin) does not carry the benefit I was thinking it might.
1- The case for making "Automatically trim safety buffer" a free feature - #9 by dreev does not specifically mention performance issues, just that there is something undesirable about many road matrix entries.