Normally favicons are very tiny graphic files, just a few KB in size.
But to my surprise I saw that the IBM Domino favicon was a whopping 449,4 KB in size . and took over 0.7 seconds to load
The reason is that an ICO file can contain many images inside and thus be a multi-purpose graphics file.
Which can be great at times.... but sending a half megabyte favicon is probably not what you want
If you want to keep the IBM Domino favicon but only a 16x16 version, you can just download the normal IBM Domino favicon from the server, open it in GIMP, delete all the layers except for a 16x16 image and save it again.
Upload it to the Domino server again to "Domino/html/favicon.ico".
If on Linux remember to change ownership.
After this change the file was 1.1 KB and loaded in an instant