If you are running WAVE remotely, a slow network connection may be the reason for sluggish performance. If possible, run WAVE locally, or get a faster network connection. You might also try changing the time scale so that fewer points need to be drawn on each screen.
If you are using WAVE to view a record via FTP, it is currently not possible to begin until the entire record has been downloaded; for long records over slow network connections, this can take a long time. Some web (HTTP) servers suffer from the same limitation. If possible, use a web server that supports HTTP range requests, such as Apache, or download a copy of the record to your local disk.
PhysioNet and its mirrors support HTTP range requests, so if you experience this problem when reading data from PhysioNet, the cause may be network congestion; in this case, try using a different mirror.