Safari (High Sierra) slow to open new tab from existing tab with editor

I've noticed a problem with Safari (version 11.0.2 on High Sierra, but also on earlier versions) where it is very slow to open new tabs, but only when I'm on a page that has a text editing control open.

For example, on this page on Apple Support Communities, where there's a rich text editor, pressing Cmd+T takes about 10 seconds to open. While this happens, Safari shows the beachball/SPOD cursor.

If I first switch to a different open tab, one without an edit control open, pressing Cmd+T makes a tab appear instantly.

The delays also seem to increase as I have more open tabs in my browser. I currently have about 60-70 tabs open, many of which contain open editors. (I do a lot of online collaboration at my job.)

I'm looking for the best way to report this as an issue to Apple, as it seems like a reproducible performance issue across all sites that use HTML editors. It happens on this website, Stack Overflow, Twitter, and everywhere else I've seen an HTML editor. That seems like something which might be possible to fix in Safari, which would be ideal, because the delay in opening a new tab is crazy-making.

NB. I have no extensions installed in Safari.