I would agree with John about voltage drop. You can buy a relatively good digital volt ohm meter, fairly inexpensively. Check the voltage at the battery, and at the big cable connected to the starter. If there is a difference, check your connections. If that doesn't change the voltage drop, the cables are probably the culprit. This would cause a slower cranking speed, and could account for the white smoke. If that isn't the problem I would look at compression. If this is the case, the first thing I do is a valve adjustment. Since diesels fire off of compression, then adequate compression is critical. Valves that are slightly tight can make a big difference in starting, as can piston and/or piston ring wear. I normally won't tear down a running engine until I do some diagnostics to determine if it is necessary. Diesel engine compression testers, and cylinder leakage gauges are effective diagnostic tools, and in recent years have become less costly. These have saved me a lot of money in unnecessary tear downs. If you do need to rebuild, the biggest problem I see on diesel engine rebuilds is resurfaced valves. That is OK on gasoline engines, but in diesels it tends to reduce the compression. Since it is compression that fires the combustion, good compression is to be taken seriously, I always replace the valves on diesels, every one, every time.
Take care!
Andy