Slew rate is a measurement of how fast the signal voltage changes over a specified period of time. At high data rates, you need a fast slew rate to ensure that the data is received accurately. At low data rates, an unnecessarily fast slew rate can introduce EMI (electro-magnetic interference) on the signal lines.
These asynchronous serial adapters have the ability to operate at very high data rates exceeding 1 Mbps. When you are transmitting data at data rates lower than 250 K bps, enabling slew rate limiting in the Windows driver will slow down the signal voltage transition, also referred to as edge-time, and reduce the chances of EMI in your application.
The oscilloscope trace in the first image shows a serial signal with slew rate limiting disabled and the second image shows the same serial signal with slew rate limiting enabled.
Figure 1. Slew rate limiting DISABLED
Figure 2. Slew rate limiting ENABLED