Hi there. I encountered lag while recording an AVI on the latest stable DOSBox (0.74). I don't know for sure if this is a DOSBox bug or if it's something system-related, so I figured I'd record a video showing what's happening and let you decide.
This video shows what's happening. Basically, this happens every 30 seconds (I timed it; 5 consecutive lag spikes were each around 2110 frames apart).
I heard on IRC from Harekiet that it might be due to rebuilding the index every n frames. I guess that's possible, and the lag did seem to get progressively worse, but only slightly worse. And maybe, if rebuilding the index is necessary and can't be avoided, emulation should be paused entirely so that the lag doesn't show up in the file.
PS: afterwards, I restarted the game, turned on AVI recording and had it play demos while I watched the Activity Monitor. The lag appeared but I couldn't see any other process stealing CPU time in particular. I think DOSBox's CPU usage momentarily went down during the lag, but it never ceased being the most expensive process.