Slow startup is caused by /usr/bin/login, which, for reasons unknown, loads those .asl files at startup.

Solve the problem by setting Terminal to use non-login shells. Preferences -> Startup -> Shell Opens With: /bin/bash

Before this, Terminal startup was >10 secs, freezing the whole system. Now <1 sec again.

1) Christophers’ way: cleaned up logs
sudo aslmanager -size 1

2) prellers’ way: this solved logs accumulation problem for future. Followed instructions at: http://www.reddit.com/r/apple/comments/lzchr/speedup_terminal_by_deleting_log_files/

Everything works perfectly. Terminal launches in microseconds now. Thank you guys for the analysis and solution.

sudo aslmanager -size 1

It will leave a file or two, but those are, it seems, the ones currently in use, and it makes sense to keep those.

The man page also says aslmanager runs regularly and you can set set max_store_size in /etc/asl.conf to avoid having a cron job or fiddling around with launchd scripts, but I haven’t tried that.

Here’s the post:
Every time you launch a terminal window or tab, at first a “login” process is started which itself starts your shell. The good thing is that “login” is open source and everyone can check why it’s doing that (and maybe fix it, lol): http://opensource.apple.com/source/system_cmds/system_cmds-541/login.tproj/

In detail the login process is started with following arguments:
login -pf

This login process reads for whatever reason every log file which is stored in /var/log/asl.
You can check that with: sudo opensnoop | grep “/var/log/asl”

So, the more log files are stored there the longer it takes to start up a shell in your terminal.

I just found out that if you change the “Shells open with” preference (http://cl.ly/BWyZ) in your terminal settings to “Command (complete path) and put in your default shell path into the textbox “/bin/bash” (it’s /bin/zsh for me, but that doesn’t matter) the login process is started with the following arguments:

login -pfq

The -q argument forces quiet logins. Those log files aren’t being opened again, you can check that with opensnoop again. The only difference is that you won’t get those fancy “Last login: Fri Nov 4 12:33:34 on ttys011″ Messages anymore. I think that’s a better workaround than deleting those log files every now and then.
I hope everyone understood me, my english is really bad…

Use -r not because you use it every time you delete something. Try getting used to not use parameters that make no sense in for the current intended action.

And for deleting files adding parameter -r is pointless. It just adds potential damage i.e. when using a fileglob that accidentially matches more things that you intended to.

find /private/var/log/asl -type f -name ‘*.asl’ -ctime +7 -print0 | xargs -0 rm -f

(will delete all _files_ (no dirs) named something.asl that are older than 7 days, and will not have problems with strange characters (spaces!) in filenames, subdirectories, or a lot of files)