Quickly checking the source, there seems to be a bug when acking interrupts. intreq should be written twice, or the interrupt can be executed twice. With vertical blanking interrupt that'd mean double speed.
Code such as this:
MOVE.W #$xxxx,$DFF09C
NOP
RTE
...should be:
MOVE.W #$xxxx,$DFF09C
MOVE.W #$xxxx,$DFF09C
NOP
RTE
No, I am not interested in fixing and building the app (the source is in m68k assembly, with spanish comments). Just something I spotted.