I've seen this ever since Safari 8 on OS X 10.9.5, OS X 10.10.x all versions with Safari 8 and sadly in Safari 9 on all El Capitan betas to date, too. The Safari memory leak in my case is severe and Safari has to be completely quit & restarted often. It only seems to happen if you tend to have a few windows open a lot, which you "reuse"; but overall, Safari just grows and grows (by many GB).
Suggestions about "putting in more memory" are absurd. I've a 16GB Macbook Pro laptop which is the maximum configuration of soldered-on ("pro" my rear-end!) memory which Apple provide. It simply isn't possible to add more. Memory pressure and slowdown tend to get critical when Safari exceeds 10GB. I did once persist to the point where it was using over 13GB. When restarted with all tabs manually revisited to ensure all pages are loaded, it'll go back to about 2.5Gb. A leak of that size is utterly indefensible.
This is a stark change in behaviour from Safari 7, which behaved basically fine in this regard - yet there are surprisingly few reports of it online. It isn't a subtle problem and Safari 8 has been around for ages. Others would have noticed, yet few report it.
I see it on my 10.9.5 machine, 10.10 home laptop, 10.11 test laptop and even, more recently, a 10.10 laptop at work. My conclusion is that this must be Safari screwing up when particular bookmark, cookie, cache and/or other data is present and this data must be part of the stuff it shares over iCloud - otherwise I would not have expected my independently clean-installed-by-IT-vendor work laptop to exhibit exactly the same behaviour.
Bottom line is that this seems to be a user data thing. Taking a deep breath and doing a complete Safari reset - ditching your iCloud bookmarks, emptying everything from every Safari instance on the iCloud account, deleting ~/Library/Safari and so-on - might work according to the Developer Forums. But as ever with Apple since roughly OS X 10.7, its a heisenbuggy mess and no amount of psuedorandom chicken slaying will be guaranteed to fix your issue.
Closed tab stray processes might just be down to a "bad extension", but that's no excuse - extensions are under Safari control, and a bad extension should never be able to break the browser. It's just JavaScript code executing completely under the browser's oversight. Still, we know that Safari must have very poor code for extension support given the problematic history, so that's always worth investigating if you haven't already.
Spotlight is re-indexing after the upgrade, which means it's busy all the time, impacting performance and battery life. Never try to judge the performance or battery impact of any major OS X upgrade until the machine has been turned on but idle for at least a week, to give time for the re-indexing to complete.
Best Answer
The Spotlight Web Content process is part of Spotlight Suggestions. When you start to type in Spotlight or the Safari address bar, Apple sends what you are typing out to their servers to suggest possible web matches and pre-fetch web pages based on what the system thinks you might be looking for.
For example, if you start to type N E, it may pre-load netflix.com.
As far as why it using so many resources, that's difficult to say. Is this the first time this has happened? It may have crashed. Keep an eye on it and see if the issue reoccurs.
You can also disable Spotlight Suggestions and that should stop this thread from appearing altogether.
Go to System Preferences > Spotlight and uncheck Spotlight Suggestions and Bing Web Searches. Then, in Safari, go to Preferences > Search and uncheck Include Spotlight Suggestions.