Hello Goran,
your problem is made by yourself.
The only way is CPLD or a Shiftregister.
But when the resource are not present nobody can solve that problem.
I have build with a CPLD a 10MHz serial transmitter with an adress interface to the 8052 and a interrupt logic.
So I have an interrupt (full working 10Mbaud) with 833 KHz. And it was in my Aplication the limit.

Need more performance ?
You must use a 16 Bit or 32 Bit Controller.

Sascha