There appears to be a bug in Jamba (probably event handling) that has a drastic effect on load times.

In testing with CPU profiling on, Jamba increases login and UI reload times by ~15 seconds for me, which turns into a lot of waiting when logging multiple characters in simultaneously. I tested this both with very few addons other than Jamba, and with all my addons enabled. UI reloads take a couple seconds without Jamba, and around 20 with. More careful testing shows that the base Jamba addon doesn't take long to load, but every module increases load times by 1-2 seconds.

I found an effective workaround too: Editing the .toc for every Jamba module to support AddonLoader. Jamba doesn't seem to have any issues with being loaded on demand other the minor problem of Jamba's additions to the quest log starting out visible, and my load times are much faster after doing this.