Is it even possible to give a definite answer to this question? There are a lot of factors involved in this like Earth's position and motion relative to the planets and of course their own motion and apparent sizes in earth's sky.
Or is it that obvious that Venus transit will be longer because its orbital speed is less than Mercury?
This is a cool question; I'll give it a go.
Or is it that obvious that Venus transit will be longer because its orbital speed is less than Mercury?
No, nothing here is obvious!
tl;dr: Times for the transit of Mercury can range from 5.37 to 8.30 hours, and for Venus they range from 7.94 to 8.12 hours under assumptions of coplanarity but allowing for eccentricity.
Let's assume all orbits are in one plane. We can get the orbital speed of body $i$ with semimajor axis $a$ and current distanc $r$ from the vis-viva equation;
$$v_i = \sqrt{GM\left( \frac{2}{r_i} - \frac{1}{_i} \right)}$$
where $GM$ is the standard gravitational parameter of the Sun which is about 1.327124E+20 m3/s2.
a ecc pei apo v_peri v_apo
Earth 1.49598E+11 0.016709 1.470984E+11 1.520976E+11 30286.6 29291.1
Venus 1.08208E+11 0.006772 1.074752E+11 1.089408E+11 35258.8 34784.4
Mercury 5.79091E+10 0.205630 4.600125E+10 6.981695E+10 58976.4 38858.6
Imagine Venus and Mercury crossing a line fixed at one end to the Sun and the other end to the center of the Earth. The linear speed of a planet $i$ relative to that line will be $v_i$ minus the speed of the line at the distance of planet $i$ or $v_E r_i / r_E$, and the angular velocity seen from Earth will be that divided by ($r_E - r_i)$ or
$$\dot{\theta}_i = \frac{v_i - (r_i/r_E) v_E}{r_E - r_i} = \frac{r_e v_i - r_i v_E}{r_E^2 - r_i r_E}$$
(seconds) Venus Mercury
Earth peri apo peri apo
peri 28570 29242 19335 29884
apo 28060 28626 19384 29646
(hours) Venus Mercury
Earth peri apo peri apo
peri 7.936 8.123 5.371 8.301
apo 7.794 7.952 5.384 8.235
