Usually optimizing for dial-up will result in certain throughput degradation on your LAN segment. Depending on Windows version, quality and speed of your local network, and other parameters, this degradation could be more or less noticeable.

In Windows XP and later, it's possible to modify TCP receive window and MTU on a per-adapter basis. First, make sure that if you use the Optimization Wizard you selected the right network adapter (your modem) when you optimized for dial-up network access. Then, open Advanced Optimizations window and on the Network Adapters tab select your LAN adapter. Reset both MTU and TCP Receive Window (on XP only) to "Windows Default". Doing so, you'll get optimal throughput on both networks.