We spent the day at Universal yesterday. We arrived at 8:45, bought our park-to-park tickets and headed into IoA. Went straight to WWoHP, the line for FJ was posted at 30 minutes, but I really don't think we spent that long in line. We went through the long line, as it was our first time, but then went right back on through the single rider line. We then walked to the Three Broomsticks, (btw - lineup for Olivanders was about 30min) checked the time, and it was 9:38! The lineup for TB was long, so we went to explore the rest of IoA, and US. Everything was super quick - we didn't wait more than 15 minutes all day, but we also did single rider lines whenever we could. We went back to IoA around 4pm - crowd levels seemed to be the same as they were in the morning. We went to Three Broomsticks for dinner, there was no wait whatsoever, but it started to get busy as we finished. We got in line for Olivanders - it took us 30 min to get through, and they only did 1 wand fitting (IMO - not worth it unless you're the one that gets picked). There was even a line to get in Dervish and Banges on the other side. We went and rode FJ another 3 times using the single rider line (all told we rode FJ 5 times yesterday).
Probably my best piece of advice to conquering this park is to avoid having bags with you. We kept everything in our cargo pockets and just walked on everywhere. Take turns and split up to go on the single riders line. One person will have to wait with your stuff, but it will still be much quicker.
Oh, and definitely try the Butterbeer!