You sir, are correct in that. Slim also merits a looking at, but I write everything [static] in Jade first and then transfer the output to whatever, yeah, JSP's. There's a significant performance hit from using non-HTML at run time, +40% in some cases, as was Haml back a couple of years ago. Probably a good reason why RoR adopted Sass yet shied away from Haml.
The Ruby slim templates get compiled to an intermediate language (Temple). That is then compiled into an end form and everything is cached - the performance is all the same.
http://jade-lang.com/.
Jade makes HAML look like a dogs breakfast.