Which G5? Which Video card? How much Free Space on the Hard Drive? How much RAM Memory?
Open Activity Monitor in Applications>Utilities, select All Processes & sort on CPU%, any indications there?
How much RAM & free space do you have also, click on the Memory & Disk Usage Tabs.
In the Memory tab, are there a lot of Pageouts?
At the Apple Icon at top left>About this Mac.
Then click on More Info>Hardware and report this upto *but not including the Serial#*...
Hardware Overview:
Machine Name: Power Mac G5 Quad
Machine Model: PowerMac11,2
CPU Type: PowerPC G5 (1.1)
Number Of CPUs: 4
CPU Speed: 2.5 GHz
L2 Cache (per CPU): 1 MB
Memory: 10 GB
Bus Speed: 1.25 GHz
Boot ROM Version: 5.2.7f1
At the Apple Icon at top left>About this Mac.
Then click on More Info>Hardware>Graphics/Displays and report like this...
NVIDIA GeForce 7800GT:
Chipset Model: GeForce 7800GT
Type: Display
Bus: PCI
Slot: SLOT-1
VRAM (Total): 256 MB
Vendor: nVIDIA (0x10de)
Device ID: 0x0092
Revision ID: 0x00a1
ROM Revision: 2152.2
Displays:
VGA Display:
Resolution: 1920 x 1080 @ 60 Hz
Depth: 32-bit Color
Core Image: Supported
Main Display: Yes
Mirror: Off
Online: Yes
Quartz Extreme: Supported
Display:
Status: No display connected