https://en.wikipedia.org/wiki/Atmosphere_of_Venus#Induced_ma...
Additionally the current theory is that if Venus will be sped up to below 56h per rotation it would regenerate it's intrinsic magnetic field which should be near earth levels.
And while Venus does have higher gravity than Mars, it's still lower than Earth's, and overall the delta V for venus insertion orbit is slightly lower than what you need for Mars.
That's great and all, but how exactly do we realistically speed up the rotation of a _planet_?
The mass of the venusian atmosphere is high enough for this to actually work and still leave you a lot of atmosphere to work with, it will take 20-30 years but it still doable.
But compressing acidic gasses makes the corrosion problem worse, doesn't it? How do you deal with that?
This is planetary engineering. It's not supposed to be fast.
Unless, of course, you are a Kardashev II civilization.
As far as terraforming goes, Venus is just a better candidate for long term colonization.
Mars' soil contains tons of perchlorate at levels which are very toxic to humans.
TBH Venus seems like a better candidate for long term terraforming.
You can build floating complexes in it's upper atmosphere, and it has enough oxygen and water locked in it's atmosphere and we might be able to fix the rotation and the density of the atmosphere in one go by using the atmosphere to speed up the rotation of Venus. While both of these are quite "far fetched" technically, as far as terraforming goes we aren't any closer to terraforming mars than Venus, and at least for Venus we don't need to figure out how to give it a magnetic field.