Why is Serial.Write that slower when writing x+1 characters than when it is writing x characters?
The Arduino core has a 64 byte transmission buffer.  Characters are sent out of that buffer by the hardware.

When you blast lots of characters out of serial the first 64 just get put into that buffer - almost instantly.

Once that buffer is full your sketch blocks until the hardware has sent a character to make room.

Since it's sending right from the moment you place the first character into the buffer you get a little more than 64 bytes worth of buffering time (64 + the number that have been sent by the time the buffer fills).

From that point on you are entirely at the mercy of the baud rate you have chosen.

To get more predictable throughput (and maximise the speed) you should bypass the Ardiuno core and manipulate the UART registers directly.