Maximum reliable speed of GPIO pin given only capacitance and no resistance
The datasheet specifies output high current of 60uA at 2.4V for ports 1-3, and 800uA for port 0 (but only in external bus mode). From this you can calculate the equivalent resistances. 5.0V-2.4V = 2.6V. 2.6V/60uA = ~43k&ohm;. 2.6V/800uA = ~3.3k&ohm;. 

In I/O mode Port 0 is Open Drain, so external pull-up resistors would have to be applied. What is the lowest value pull-up resistor that still allows the pin to pull down to TTL logic 0? Port 0 pins can sink 3.2mA at 0.45V. 5V-0.45V = 4.55V. 4.55V/3.2mA = ~1.4K&ohm;. The other port pins can sink 1.6mA, so they could have external pull-up resistors as low as ~2.8k&ohm;.

The 89S52 needs 12 clocks per machine cycle. At 22.1184MHz this corresponds to 0.543us. 43k&ohm; x 10pF = 0.43us, so even the weak pull-ups on ports 1-3 should be strong enough to toggle at full speed. However anything connected to a pin (even a scope probe) will have its own capacitance which will increase the rise time.