Only time my planes get slow is when they are REALLY big and I pan around alot. That'd prolly be because of my compy. Kinda weird though, since rendering and all other tools and stuff work normal speed, but big planes just slow it down :/

2.8GHz Pentium 4
512 MB RAM
Intel Integrated GFX card (lol)

Didnt exactly buy it with 3d animation in mind, but nothing a few upgrades cant fix.