works fine here, seems you got a buffer setting wrong somewhere in Reaper.
Don't know how other DAWs work, but Reaper keeps the plugins going _all the time_, unless you mute a track (this can be configured of course, but this is how it is default). That means that putting 80+ instances of Nebula for example can make your whole system crawl because (Even if stopped) Reaper is still letting the plugins calculate stuff. Just as a note in case you were working with lots of instances.
But a new project, single track and loading Nebula on it: less than 2 seconds easily. Only the very first time (after upgrade for example) when it scans its complete database it can take up to 20 seconds. After that I can close reaper, start it again and load Nebula again on a track all within 4 secs .
I have no clue but it seems something is not OK on your end, cause it works fine here (Windows 7 SP1 x64, Windows 8.1 x64, Reaper x64).
Did you try running Reaper in 'administrator 'mode (right click on the shortcut, then 'run as administrator'). Who knows, maybe Nebula doesn't have enough rights to build it's database and needs to redo it every time. Just guessing here.