Trouble interpreting iostat's tps measure

That's a single iostat invocation, which does not provide meaningful data for "per second" values, only counters are useful - it can not calculate change per second from a single value, it needs to know two values and time between them. To see real values, try something like :

iostat -d 1 2

The second output will be real values.

From the iostat manpage:

The first report generated by the iostat command provides statistics concerning the time since the system was booted. Each subsequent report covers the time since the previous report.

Since the transfer rates are the same, the output you provide says to me that the MD layer caches transactions and then writes them to the physical disk in larger chunks.

The slowness you're seeing with reloading the database is likely due to indexes, as you said. You can avoid this by just not having the indexes update as you load the data, then just recreate the indexes at the end.