I don’t think this issue is really resolved. My schematic has become pretty slow to respond as it’s grown in size in all the browsers I’ve tried: Safari, Chrome and Firefox. I’m using a new 5k iMac with a Core i7 and 25Mbps FIOS. The issue affects larger schematics, particularly when moving modules or components near modules. The lag is on the order of several seconds, making it nearly impossible to work on the schematic. I have to agree with the original poster, this makes Upverter largely useless for anything but small/hobby boards.
For reference, my schematic is here. Panning around is slow in Safari, ok in Chrome and Firefox. Moving components on the other hand, especially near modules, or moving modules themselves, is terribly slow in all browsers. https://upverter.com/UltralyticsLLC/2ec721a77f55b77b/DRS4-Carrier-Board/