I placed a 2x2 minecart track under 8 dispensers and put maybe 15 or so normal minecarts on it. I then had the dispensers simultaneously place 8 TNT carts onto those 4 tracks. The game instantly slows down to 1 fps and eventually freezes and stops responding. The crash report says one of the tnt minecarts has
Momentum: -3439244417376555000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00, 0.00, -4961543663356229300000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000.00

seems like a lot to me, especially if it explodes and deals damage based on speed.
When I reload the map after the crash I spawn where the screenshot is taken from. (I took the screenshot before pressing the button)