When you do a global search across your library, the Kindle tends to slow down the more books you have -- but I would expect (without any knowledge one way or the other) that the Kobo is the same -- searching all files is bound to be more resource-intensive.
Searching Wikipedia on the Kindle is seamless, highlight the word/phrase via a tap, and you will get a few options. Default is to show the dictionary, but you can also tap "more" and then "Wikipedia" to search Wikipedia for the word/phrase. I don't know if the Kobo does this, my Google-fu turns up nothing.
self-generated X-Ray files are fairly simple. The whole thing is a total hack, but well-maintained by the MobileRead community here:
https://www.mobileread.com/forums/sho...d.php?t=245754
The new SQLite format for fw>=5.6.0 is still being tested out though.
You cannot do the same with Kobo.
Some effort was put into it (the Beyond The Book) by the Kobo hackers here, but ultimately it turned out to be unfeasible and the project was scrapped.