"it's taking longer than what I expected."

As Steve asked, how long is it taking?
How long did you expect, and on what basis?

Are you talking purely about download, or also Flash erasure, programming, and verification?

What speed are you running the serial port?

Is the bottleneck your 8051, or your PC?

What format are you downloading - Intel Hex? Binary? or what?

Are you using flow-control?

Is your serial IO interrupt-driven?

Are you using a protocol with big overheads?


etc, etc, etc, ...