A side effect of using a smaller maximum segment size is that
`scm_i_adjust_min_yield ()' gets called more frequently (from
`scm_gc_for_newcell ()'), which explains the "smoother" increases of the
min yield.

Using the script I posted earlier, I get the following execution times
for 2500 iterations:

  * max-segment-size = 64 KiB

    real    3m1.167s
    user    2m56.200s
    sys     0m2.220s

  * max-segment-size = 20 MiB

    real    3m29.047s
    user    3m22.392s
    sys     0m4.764s

Thus it would be sensible to change `SCM_DEFAULT_MAX_SEGMENT_SIZE' to
some smaller value.

Thanks,
Ludovic.