> I am currently downloading a large website which is
> averaging a download rate of about 20Mb an hour.

It depends on the remote site capacities, and average 
bandwidth. Note that the default bandwidth limit for 
httrack is 25KB (the 25000 setting in "limits"). Besides, 
using more than 4 connection is generally not necessary 
(and it can slowdown the remote server, limiting the total 
performances)

But 20MB/h is a bit slow, that's right - maybe you are 
downloading a lots of dynamic material, or material with a 
lots of tests ?