If you only have the minimum requirements that can be the reason, you should try to exceed them in some way, and remember that PS is far from having a pretty optimized code, so, really some places do lag a lot.
What you can try?
One of the things is, editing the shortcut that launches planeshift (right click on it, then properties) and change where it says (...)psclient.exe\" for (...)psclient.exe\" -fullbright (please note the space between the \" and -) If I\'m not mistaken that\'s said to give some extra fps.
What to try next?
You can ensure you aren\'t running any useless programs in the background, you can also play disabling windows services to free up even some more resources, trying disabling antivirus, or adding a more lax rule for your firewall software (if any) so less cpu load is done...
Good luck.