I can confirm that I have been having the same problem for the last 48 hours approx. Safari and macOS Mojave.
I thought it was me so I've run all sorts of tests and OnyX, rebooting, Recovery first Aid on SSD etc. (I didn't need to do all that as this only applies to the Forums!) It really is very, very slow to open a window.
Oddly, the Forum windows opened closer to normal with Brave.
Tricky one to solve if the only two who have mentioned this are in the UK and in my case, it is faster on Brave. But Safari has till now been super fast.
Your assistance requested
Ian