Twenty seconds is a long time to crank. Certain fuel supply issues can delay starting as well. I almost would bet you have not checked for reasonable valve clearances. In todays world many people take it for granted they are self adjusting or not important.

I always like it when the really cold weather starts. There are multiple glow plug responses. Yes they frequently are the problem.

If you do not have a decent volume of fuel spraying when you hit the key. It cannot start until a supply builds. If for example air must be purged first or for some reason the initial supply of fuel is at a substandard pressure to the injection pump.

All too many times people just think the car is old. The reality is it has never been properly serviced for perhaps a dogs age. In general this is a very cheap thing to do yourself.

Today a lot of commercial service establishments would not know how. To many current working mechanics they are a blast from the past.

This is just one reason to properly check out the fuel supply system in good weather. It improves reliability and extends the life of the starter and battery.

Especially when you heavily over discharge the battery when attempting to start. The average modern car battery really does not like that at all in my opinion. When you have series loop lugs.

Carry a small meter and some item so you can bypass a burnt out glow plug. I have had a couple of diesel engines that just would not start hot or cold without some glow plug function. Otherwise they appeared normal. I never trouble shot that particular issue either.