I believe you will see a difference if you re-generate the output and change the speed option from Local PC to the opposite setting. With Local PC, you are forcing the TOC to fully download before it's presented. This is likely causing the delay. The opposite setting causes the TOC to be loaded in smaller segments so it appears to load faster. The problem with that approach is that the synchronization may not work straight away because you may link to a second or third level topic that loads and tries to synchronize before its segment has arrived.
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I'm experiencing the same latency in the TOC, but I am generating AIR Output from RoboHelp 8. Can you tell me if there are comparable settings to adjust in the AIR settings?
Just an update, changing from "Local PC or Intranet" to "Website (Internet)" reduced the TOC load time from about 8 seconds to 4 seconds. Still doesn't appear instantaneous like on my other RoboHelp sites that do not have merged projects, but suppose 4 seconds isn't too bad and will have to live with unless there's some other further settings to speed it up.
The TOC from my previous site with the same number of merged projects and topics loaded instantly but we were then publishing to RoboHelp Server, so guess that would be one solution....