Why does this character render so much faster than other?

A lot of characters now use chromatic sss, like Ensley does. Ulrike uses mono sss.

Chromatic sss is a massive performance hit. Just toggle Ensley to mono and Ulrike to chromatic and you will see a big swing in render times.

The more different surface settings you have, the more complicated it becomes for Iray. If you have translucency, that adds a lot, but of course skin needs that if you want it to look real.

Chromatic sss is somewhat new. Iray did not have this until like Daz 4.10. So all the characters made before that used mono. You will find that most of those characters render much faster than any chromatic based character today. I noticed this right away when I got my first characters that used chromatic skin. On my old machine they took so much longer to render, that I avoided using characters with chromatic until I got faster GPUs.

So if you are looking to render faster, try to use mono sss if you can find a setup that looks good. That is the catch. Chromatic kind of gives skin that little extra variation that mono lacks, so it can take some tweaking to get mono looking good.

I think it is because the wet surface covers the skin. Think of it as like clothing in a way. You might notice with some clothing your render times actually drop.

Texture size should not effect render speed that much. It mainly effects VRAM. Also, Iray has a built in compression that automatically compresses texture files over certain sizes. You can set these in the advanced setting of Iray. There are two, one for medium threshold, and one for high threshold. Textures over the medium get a medium compression, while ones over high get highly compressed.
By default, these settings are shockingly low, so if you've never changed this setting, you have been compressing those textures all along. In other words, the size didn't matter at all in the end.
Sometimes it is good to increase this setting to get a better quality render. But if the texture already sucks it wont matter.

The textures are not compressed in RAM. So your active scene always uses more RAM. This is why your RAM is always higher than what you use in VRAM for rendering, that is Iray's compression at work. It is kind of scary to think with how much VRAM Iray consumes that it can actually be even worse without this compression.