Low speed is anything like dial up modems, such as the 56k modems. High speed includes DSL and cable modems. It is not going to hurt you to run a low speed test on a high speed connection, but running the high speed test when you have a low speed connection will take a few minutes to complete.
The selections are available only to help speed things up. For very high speed connections, the high speed test will provide greater accuracy than that of the lower test.
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