To what degree should the oldness of my phone affect my tethering speed?
Bluetooth is slower than WiFi but it conserves battery power.

BT are capable up to 2 Mbps, which equals 240 KB/s, sounds like you are right on the target.

Kilobytes per second (KBps) and Megabits per second (Mbps)

Source:


  Bluetooth 2.1+EDR, the flavor built into all iOS devices, has a raw
  rate of 3 Mbps and a net throughput that’s just a bit over 2 Mbps.
  That’s fine on Verizon’s 3G network, where average speeds never top 2
  Mbps.