Earth's eccentricity, e, is about 0.0167. If a is the semimajor axis and the average speed, u = sqrt[GM/a].

Then the velocity at perihelion is:

V = u*sqrt[(1+e)/(1-e)]

and at aphelion is:

v = u*sqrt[(1-e)/(1+e)]

Thus v/V is (1-e)/(1+e), which in this case is 0.967. Not a big difference, but when propellant mass increases exponentially it can translate into a lot of payload."

So, with Earth's average speed orbital of 29.79 km/sec, it is:
perihelion: 30.8 km/sec
aphelion: 28.81

Or difference of 3 km/sec.
That is more than I thought it would be.

It makes sense to launch from Earth's aphelion for Hohmann transfer orbit, but I think using Interplanetary Highway to get to Venus one would instead leave Earth's orbit at it's perihelion.