We got massive lag last night because our chat log was so long.
Part of the reason being we had multiple bad guys (over 15) having to roll 3 attack rolls, and that we play high combat for location and weapon damage. So all these rolls were clogging up the log, aside from the normal banter, rolls for saves, percentile, etc... thank god for Macros!
When we reset the board, by activating an older VTT we had in Google+ Hangouts, then pressing the ROLL20 app again, the chat was cleared, and the lag went away instantly.
I would love an in game archive/reset the chat log button, without the fear of losing the whole board, if say our internet connections suddenly decided to wonk out... then our reset trick would not work.