If your movie plays OK on your computer and the lag problem exists only while streaming it to the TV via HDMI cable - this will not be a proble with iTunes.
This will be computers or TV weak hardware. Also make sure you use HDMI cable at least 1.4 as older cables have lower transfer and can cause problems.
Let us know more details if it needs more troubleshooting.
The HDMI cable is from my xbox and my tv is a brand new plasma LG model. My laptop was is a tripple core processor and there isnt anything wrong with it. Ive streamed netflix from my laptop to my tv the same way and don't encounter that problem, its only with itunes. Even playing a blue ray disc in my laptop to my tv doesnt lag.