This is specifically about its behaviour with "Minecart Improvements" experiment turned on.

Even with the default speed you will notice that the furnace minecart push speed is slower than what a minecart can achieve on it's own. This can especially be seen at high minecartMaxSpeed values because the push speed stays painfully slow.

Reproduction Steps

• Set minecartMaxSpeed to 1000
• Build a powered rail track
• Place a cart and a furnace cart behind it
• Power the furnace cart

Observed Results

• The pushed minecart moves much slower than had it not been pushed
  high speed example.mp4
  default example.mp4

Expected Results

• The pushed minecart would move just as fast if not faster.