Its safe to remove the *.asl files after some time, to ensure that your terminal loads fine.
$ cd /var/logs/asl
$ sudo rm *.asl
It would be good to do a backup just incase you need to trawl thru history. So on your desktop, just create a folder and move all the files into it
$ sudo mv *.asl ~/Desktop/myasllogs/
This should hopefully shed a few seconds off your terminal loading time.