After moderate testing I made some conclusion, but seems very fragile to me. I made testings with root file system settled at NFS and other time in SD card. Exact starting times are with NFS: xterm - after 12 sec and xclock, after 70 sec but with SD card: xterm - after 12 sec and xclock after 90 sec.
As my ARM board has 64M RAM I consider that problem is in swapping time. That would explain differences between NFS and SD card startup time (ethernet is 100Mb, SD card - 25MHz). However question exists- why difference in starting of xterm and the xclock. My only (faint) explanation is that maybe xterm uses direct X server library calls (and X server starts after 10 sec so it's libraries are then available), but xclock program is build for GTK, so GTK libraries need to be also loaded in RAM and because of that swapping exists.
Does it seem as logical explanation to anyone (not so to me)?
However, I must confess that even 30 sec sounds as pretty long time to me (CPU= 400MHz), and above mentioned times don't include kernel booting time (which is "acceptable"). Just for comparation - Windows CE 6.0 needs 7 sec (from power on) on that very same board to show desktop with icons !
I appretiate any comments.