MacOS – Why is the G5 running so slow and hot

You could try FireWire Target Disk Mode. Boot the source computer while holding down the T key and connect to the destination computer. The source's drive(s) will show up on the desktop and you can copy files that way.

I don't know if it will be any faster; the bottleneck could be your hard drive speeds.

The Core i5 processor will throttle it's speed if it gets over what Intel refers to as the 'TJ Max' which is somewhere around 90C. If your software is telling you that you're hitting 100C, then you've obviously got a cooling issue.

Unless you suspect the previous owner tried replacing the CPU or heatsink (unlikely), it might just be that the vents are clogged or the fan is clogged or not working. Fortunately, this should be easy to get to the fan as per the iFixit guide here. You can probably confirm the fan performance by putting your hand over the vent in the back when it is running hot and see if it is blowing a lot of hot air on you; I imagine you'd barely feel anything when you should feel something relatively significant (think: it should blow/flap a piece of paper you hold over it like if you used your mouth to blow air onto it).

Before buying a new fan, you could try seeing if the fan spins easily with your finger (and keeps rotating for a short moment). What I've found in the past is that these little fans can get dust either built up on the fins (easy to clean) or the bearing and/or rotation points could have dust buildup that is causing the fan to be difficult to rotate. What you need to do is clean out the dust (perhaps with some tweezers) and see if you can make it spin with more ease again. Further that, if you are comfortable enough with somewhat disassembling the fan and could get some oil into the fan's bearing, that might make it as good as new. Typical good oil for this is along the lines of "sewing machine oil" or perhaps if you have a RC Car shop nearby, they have nice oils with needle applicators that works great. Reinstall fan and things should be good.

As far as the performance degredations you're seeing and the higher CPU usage, that is certainly inline with the throttled i5 speed; if the chip is running slower, it isn't getting tasks done as fast which means these tasks are taking more of the CPU time (if you look at it as a measurement of time, not capacity).

Hope this works for you!