[LAU] Analysing (debugging) USB port performance

Hi LAU, I have anecdotally, but recurrently observed on laptops - including a rather recent one - that there is always a 'best' USB port for external sound cards. For instance, on my latest machine with a decent 'realtime audio' configuration/set-up (real-time kernel, /etc/limits stuff, 'performance' CPU governor, (wireless) network switched off), I'm able to have a pleasant xrun-free session recording in Ardour including a bunch of tracks with effects playing at 64 frames and period of 3 with a relatively cheap card (UMC202) on one of the USB ports. On the other hand, in the exact same conditions I get incidental xruns at even 128 frames and xrun instability at 64 frames on the other USB ports. I wonder: 1. Is there a more scientific (well, precise at least) method to assess this USB port performance? What to test or look into? 2. Is there a way to change (e.g. improve the not-so-good USB port performance) OS/software wise, or is this usually hard-wired in notebooks? 2a. Are IRQs relevant on laptops and if so can a whole USB port (or the device attached to it) be optimised from the OS? Of course I _can_ live with one 'good sound-card port' on a laptop but I'm quite curious about people's experiences and the gurus' wisdom - albeit on my former machine this was the left-side port which was closer to where the sound-card usually sits, now it's on the right, too bad! :) Hopefully other LAU have mused about such USB-related mysteries in the past... Lorenzo.