Detecting IR sensor at higher speed
It's not entirely clear what you're asking, but I think you're trying to receive short pulses from the W output of IC2 into the P3.0-RXD pin of IC1, and you're discovering that 2ms pulses are detected at IC1, but 125uS pulses aren't.

the 1K resistor is far too large, especially since it's having to work against the LED and its 100 ohm current limiting resistor.

The simplest thing to do to fix your circuit is to connect the LED and the 100 ohm resistor between the Y (pin 5) output of IC2 and ground (noting the polarity will be different). Change the 100 ohm resistor for something bigger (for a red led you probably want something more like 330 ohms).

With that modification you won't be loading the output excessively.

Next reduce the 1K resistor. The ratings for the 74HC151 give maximum source or sink current as 25mA, and the AT89C2051P lists a maximum source or sink current of 25mA as well. With a 5V supply, 25mA gives you a limit of 200 ohms, so swap the 1K resistor for a 330 ohm one.