I used chrome’s remote debug tool to look at what was causing the loading delay, and it says that the initiator for bmi.js from 1.2.3.4 is tweet_button.1379634856.html with
bmi_SafeAddOnload(bmi_load,"bmi_orig_img",0);
Being added to the start and end - agreed it doesn’t sound like anything Twitter would be adding, but if it’s happened across a couple of different ISPs (well at least mine and by the sound of it Markus’s as well), and also seems only to be affecting tweet_button.1379634856.html it seems pretty weird!