First off, I have shit internet. The Mail/TenFourBird apps open & display a window instantaneously, however it takes time for the content it's self to actually load. Be glad yours take only 5 seconds (P.S. what's your provider so I can switch over).
Second, I upgraded the gpu on my G5 despite the lack of support. One of the best choices I ever made.
PowerMac G5 Dual 2.0GHz 2003 (Model: 7,2)
Mac OS X 10.5.8 (Leopard)
7GB RAM (OWC PC-3200U-30330 DDR SDRAM 400MHz)
ATi Radeon X800 XT (GPU overclocked to 500MHz, VRAM to 550MHz)