The front-end interviews are a mix of front-end focused interviewers and gen-eng interviewers (usually 3 & 2, respectively). That's because the frontend-SWE position involves writing JS/HTML/CSS to run in a browser, but it also involves writing the server code that generates that JS/HTML/CSS (same reason that Java or C++ is part of the job req). We care very much about efficiency when your code runs on tens of thousands of servers.
There are also a bunch of algorithmic questions that come up in the browser as well, because unlike a lot of sites, Google actually cares about latency. You often can't use a library function if it involves pulling in a 300K library, nor can you write an O(n^2) algorithm if it involves locking up the browser.
A non-trivial percentage of visitors hit Google Search with an empty cache and no cookies. I know the numbers but aren't sure I can share them, but it's enough that relying on caching to solve latency woes doesn't work.