This is a side-effect of optimizing certain animation files for 64-bit systems. If you are running on a 32-bit system, there can be a noticeable delay and CPU spike while the data is loaded.
If you’re currently experiencing this issue and you’re running a 64-bit client (which should be the default), please submit a bug report.
If you are running a 32-bit client and find the delay objectionable, you can collapse the avatar header so that it won’t load your 3D avatar when you open the character sheet.