@Srksii
Just like @pandareno1999 said; I am fairly sure that you arent being slowed (at least not more than a normal bullet hit) when you get stuck by an Arcstar; the slow effect comes at the exposition.
And regarding the Caustic gas; It seems to me that the most pronounces slow effect for the enemy happened the first tick when they enter the gas, probably mainly because you lose all your current momentum. But when you are in the gas, I dont think the slow effect is so strong that its impossible to "strafe and jump around like crazy". Its not like an arcstar slow and honestly, it would just be stupid if it was. When the blurred vision effect still was a thing, it pretty much was a guaranteed loss to fight a Caustic in the gas. Now its at least possible, even if you are in a huge disadvantage.