The stats function plot.ecdf is fine for vectors that are not too large. However for n ~ 1e7 we would observe a dramatic performance breakdown (possibly in combination with the use of do.call).
PlotECDF is designed as alternative for quicker plotting the ecdf for larger vectors. If breaks are provided as argument, a histogram with that number of breaks will be calculated and the ecdf will use those frequencies instead of respecting every single point.
Note that a plot will rarely need more than ~1'000 points on x to have a sufficient resolution on usual terms. PlotFdist will also use this number of breaks by default.