"Hub_2" is a 5-port which (IIRC) claims that each port can independently negotiate its rate with whetever its connected to. There is no such claim for "Hub_1". ... C ... only has a 10BaseT

Switch_1 might have auto-negotiation for speed even if it's not claimed: it is very common on switches. If it doesn't, your problem is likely to be because Switch_1 and Machine_C are expecting different speeds. I don't know exactly what that would do, but I would expect collisions and hence delays.

Try swapping the switches over.

...

and are they all indicating that everything is plugged in at 100 meg?

I don't know -- how can one tell?

Most switches either have an additional light on each port when the link is at 100Mbps, or one of the lights is a different colour based on data rate.

Hope that helps, Matthew