Migration taking increasingly long periods of time
What's the total number of nodes you're importing? I've hit this issue before using the D2D Migration module. I've found that D2D starts to slow down on consecutive batches when I'm importing in more than ~5K nodes. In scenarios where I've needed to import 10K-100K nodes, batches eventually slow down to where nothing is ever imported.

Using XHProf &amp; XDebug, I traced down the issue to database objects consuming tons of memory &amp; CPU to the point of stopping PHP execution. I've never been able solve the issue for importing over 10K nodes using D2D. Searches all over the internet have produced nothing. In many instances I've had to rely on custom PHP import code as since I could not use D2D.