Given a connection speed of 56,400 Mbps and a series of Web pages (including graphics) totaling 423,000 bytes - what is fastest time for the pages to be transferred?
Each Web page is composed of multiple elements, each of which (typically) requires a separate TCP connection to be established to a server. A page composed of many small elements will likely take longer to transfer than one composed of just a few elements with the same total byte size.
Each TCP transfer involves some overhead for establishing the connection, and then a slow start as the connection determines the maximum possible transfer rate. Depending on how fast it maxes out (that can depend on several things such as packet loss, RTT to the end host), the best possible time and the actual time to complete any given transfer can be quite different. For most typical downloads (latency < 10 ms), the effect is small but can be cumulative for many connections when packet loss is present.
Best possible time to transfer would exclude TCP effects and any Layer 4-7 overhead. The rated 56.4 Kbps is at Layer 2 and Layer 3 would see approximately 50 Kbps (without compression). This would get a transfer time of
423,000 bytes * 8 bytes/bit * 1/50000 bits/second = 67.7 seconds
Without packet loss (aside from congestion) and negligible propagation delay to the Web server, this value might increase by 10-20% to include the overhead of TCP. And your actual performance is probably worse.
A quick guess at the typical context suggests
Most web browsers will initiate up to 4 simultaneous TCP connections to optimize transfers - but this still suggests an overhead of 25 TCP open/close handshakes. This will negatively impact the best performance. The presence of compression on larger elements will positively affect the performance.
So in fact, your mileage will vary. But I hope this gives you a rough estimate to work from.