Correction: It happens when you add a new talk page section (rather than just creating or editing the talk page).

Frustratingly (!) right now it seems to be performing acceptably fast, whether safemode=1 or omitted. This good performance is unusual and I haven't seen it so fast for a while. Then again I was just trying it in one tab. In a normal editing session I might typically have 10 or 20 tabs open. Could that be a factor? For example (off the top of my head) is there background AJAX stuff happening with this list that could be delayed due to other pending requests? Just a thought. Sorry I can't reproduce the slowness right now.

Yes, the custom list is slow on other pages for me too. e.g. block user, where you choose the block duration and reason.

Dell Inspiron 546, AMD Athlon processor LE-1660 2.80 GHz. 4 GB RAM.
Windows 10 Pro 64-bit. Google Chrome 73.0.3683.103.