Is C/2012 S1 (aka ISON) the fastest comet on record to-date? This source may be nebulous (no pun intended!) for some. It writes to say


  Starting at around 95,000mph at the start of this month, Comet ISON will eventually reach an astonishing 845,000mph on November 28th when it catapults round the Sun at perihelion


I'm no astronomer; the only comets I even know the names of are 


Shoemaker Levy
Swift Tuttle
Wild
Halley Bopp


To the best of my knowledge (feel free to poke me in the rib if i'm wrong!) none of the above list achieved a 3-digit velocity at perihelion. In contrast, going by the figure quoted above, S1 is expected to achieve a mind-boggling 375 km/s!

Is C/2012 S1 the fastest comet on record to-date?
A comet that travels to the Sun from the Oort cloud will be travelling roughly at its escape velocity at perihelion.

The fastest comets will be those that travel from the Oort cloud and plunge directly into the Sun.

If you want the comet to emerge from around the Sun, then its perihelion distance must be no lower than $\sim 7 \times 10^{8}$ m. At that point it will have approximately the escape velocity at that radius $v_{esc} = (2GM_{\odot}/r)^{1/2} = 617$ km/s or 1.39 million miles per hour.

Therefore I expect that lots of comets have attained something like or exceeding this speed before. Comet Lovejoy for example achieved a perihelion distance of $8.36\times10^{8}$ m and was travelling at 536 km/s (1.2 million miles per hour, and in fairly good agreement with the formula I quoted above - the aphelion was not infinity, but $\sim 157$ au, which largely accounts for the difference).