Some of the LAG is on the Disney side (server issues, etc.) and some of the LAG is on the end user side (your PC). If you are running POTC Online after a clean boot with no other applications running in the background it should run fairly well (on a PC with decent hardware).
There are many other things that factor into this. Your CPU, your RAM and your Video card are HUGE factors. It's a night and day difference from my third PC which I use for backup purposes only to my primary. I also played on my friends new Quad Core CPU Dell (running Vista) with 4 GIG RAM and a 512 PCI Express Video card.....this ran awesome.....everything was very smooth with an occasional hiccup.
Aside from tweaking your OS and running system utilities (disk cleanup, scandisk aka error checking, and disk defragmenter) it all comes down to hardware. I have not personally tested POTC Online with the slower throughput DSL (1.5 or less) services either. I'm on Comcast Broadband which averages 10-15Mps on the downstream and 1Mbps on the upstream. If your system has an open PCI/AGP/PCI Express slot and will accept a higher end video card...it's 100% worth it. Adding RAM will also make a difference as your hard drive will be less active caching data.
newegg.com is a great place for upgrades - I ordered the wrong memory for my Dell and they told me to keep it as it was not worth the trouble sending back as they could not resell it (invoice was around $27.00). That is customer service!