Sources give varying speeds for the vessel of 13 or 14 knots. 13 knots = 16 miles per hour (26 kilometers per hour) Significantly the author says the the estimate of 20 miless per hour was made 'before they try hor' Her first voyage was in September 1859

References:
https://en.wikipedia.org/wiki/SS_Great_Eastern
https://www.gracesguide.co.uk/SS_Great_Eastern