This does not happen with minecart improvements disabled.
This affects nether portals, end portals and end gateways.

Reproduction Steps

• Push a minecart towards a portal
• Use /tick freeze and /tick step 1t to precisely follow the carts movement
  bug_demo(1).mp4

Observed Result

• The minecart is frozen in place for 3 game ticks before it starts to move again

Expected Results

• No delay would be present