I've owned five TT's and all had slow USB transfer rates. Nature of the beast I guess.
8.412 is a good navcore. I've tried others on my ONE v2 but always go back. I do like the SE version better (SE 8.412.1236), so if you want to play around try it. Most of the newer navcores didn't seem to work since they were developed for models with 4.3" screens, that's my theory at least.
My original bootloader didn't support larger SDHC cards. I upgraded to 5.5112 and then to 5.5128. 5.5112 seemed to boot a little faster, so if I could go back I'd stop with it. I don't think your model has a SD card slot, so the benefits are low and the risks are high if you play with your bootloader. Stay away from 5.5130. Users say it slows the USB transfer rate to a crawl.