IMVU chat requires more computer power than most  applications.  Some computers will just not work other will require some  consideratsion to achieve an improved performance.

Things to consider

Chat Settings - use  Settings at thetop of the messenger window

In most cases -  use Direct3D and not  OpenGL

Use Target Frame rate 20 - try lower values  to make it run faster,  but so not so prettily

Try turning off  sound - if that makes a difference, you may needto change your sound card  drivers or change  settings

Other settings

Close all other programs - expecially graphics or  games

Clear your IMVU cache

Defragment your hard drive

Buy a standalone graphics card

Trying to run PhotoShop and IMVU chat at the sametime on anoder computer, can use 100% CPU and bring things to a crawl