Whenever I share my external Lacie drive (250 GB) it tends to slow the system down particularly (has over 100 GB iTunes music library.) Also LW doesn't always reliably read it, & I don't seem to get uploads from it using LW.

Just as an experiment, try moving a no. of the shared files over to the HDD. And share those "instead of" the external drive. Tell us if that makes any difference.

I've a suspicion it's related to the external drive constantly calling on the processor to read it by requests from LW & finder. Meanwhile anything else you might do on the computer will probably slow the process down & its reliability even further. And there may also be constant search hits to your files. Just a guess.