gitk performance questions/issues

Since the gitk is being discussed, I'd like to see if I'm understanding correctly and hitting problems or just not doing things correctly. (This is using a self-compiled "git version 1.5.2.2".)

[Caveat: this is on a repository that is evolves using my own scripts which mimic the git shell scripts in calling the low-level programs. I'm 99.999% sure it's generating a fully valid repo, but I might be missing something. Actual generating script is at http://www.personal.rdg.ac.uk/~sis05dst/chronoversion.htm ]

1.  With gitk --all -n 256 on my repo it consistently
takes 12s for the window to appear and 21s for the
"I'm working" cursor to change to a normal cursor.
This is on a dual Xeon machine, /proc/cpuinfo
excerpt

model name : Intel(R) Xeon(TM) CPU 3.00GHz stepping : 3 cpu MHz : 2992.496 cache size : 2048 KB

running x86-64 Fedora 7, packed repo has a 12M .git dir. Is this the expected start-up time for such a configuration? The output of time git-rev-list --parents --topo-order --all>/dev/null is

real 0m0.048s user 0m0.044s sys 0m0.005s

2. Startup time isn't an excessive problem. However,
I often don't seem to be getting the "diff with parent"
being displayed once I click on the another commit. If
I click on several in sequence I get the error
message

"error getting diffs: couldn't fork child process: not enough memory"

The diffs are relatively small (consecutive commits are generated hourly automatically) and a command line git-diff generates them instantly.

3. Is "gitk --all -n 256" the best way to say "show me
relatively recent stuff" or should I be using different
options for limiting things?

Many thanks for any insight,