OK, well I plugged in my USBtinyISP into the board under test. It has an Atmega32U4 on it, although it isn't a Leonardo. Since that is the same chip as the Leonardo I selected Leonardo in the Boards menu. (It's actually uploading using ICSP not the bootloader / USB interface).
My time to upload and verify was 45 seconds, compared to 11 seconds using my programmer. Maybe the configuration (in boards.txt) can be improved, but so far that is a lot slower. Using USBtinyISP that was only 215 bytes per second uploaded, whereas with my Uno as a programmer it was 879 bytes per second.
pollUntilReady ... that and fixing heartbeat makes the biggest difference to ArduinoISP.
Are you suggesting a fixed delay?