Increase the speed of the XL Excavator and XL Truck as they are very slow

I do find that the tier 2 trucks are so much faster and can do 3 loads in the time the big haul truck has been filled.

I don't think the specific of the issue is that they just move slow. Obviously, they have to be able to outperform t2 vehicles when run fully loaded (either the speed must be buffed or cargo size, not sure). But certainly, for mining applications they seem to be not as efficient as t2 trucks, and that is unrealistic, and that is the issue.

For loading and dumping to/from containers I think you are right. The research that increases the number of total vehicles should also increase speed.
For the mining vehicles this is mostly a non-issue when Advanced Logistics is researched. But speed would help.
The speed research would also help with harvesting trees. They are usually far away and the trucks assigned to them need to be faster.