If you are willing to slow down other things your Mac is doing, then you can increase the priority of BOINC. Use Activity Monitor (Process Viewer in older OSX versions) or 'top' (in Terminal) to find the PID of the worker. Assuming nothing else is running, the worker will be using the most CPU. Then give the following command in Terminal:
renice -19 xxxx
... where 'xxxx' is the PID of the worker. This sets the worker to maximum priority, it will run a little faster at the expense of everything else. note: you can quit the Terminal after giving that command.
If it's slowing other things down too much, you can reset it as follows:
renice -19 xxxx (maximum priority)
renice -10 xxxx (high priority)
renice 0 xxxx (average priority)
renice 10 xxxx (low priority)
renice 19 xxxx (minimum priority)
The nice values in between are valid too. Supposedly the scale goes from -20 to +20, but I tend to avoid the '20' values because they weren't always honored on past versions of OSX. They ought to work in Tiger.
You can directly set a process to a lower priority (positive numbers). However, unless you run as root, you will need to su to root (or sudo) in order to set higher priorities (negative numbers). If you aren't the root user, then you do it like this:
sudo renice -19 xxxx
... the Terminal will ask you for the root password. Type it in, and the command will execute. You may need to enable the root user account first. You do that in NetInfo Manager (in the Utilities folder). Security menu -> Enable Root User. It will ask for a password to be set. Give one, then exit NetInfo Manager.