Google maps (from "Portugal" to "Finland", not very specific, but we don't have any details) has three suggestions that they all claim tke 47 hours (if you're now slowed by traffic).

For a drive that long I don't think "as fast as possible" makes sense, you'll have several layovers, and in determining the length of each section you will probably have several things to consider. Like is it only you that will be driving or can you drive for 8 hours and then let your wife drive for another 8?

If one person has to do all the driving I think the fastest that's fairly safe (thinking about e.g. awareness behind the wheel after many hours of driving) is taking the route through Denmark and Sweden (that's the shorter one, and gives two breaks on ferries) and cut it into 4, that suggests a layover somewhere in the middle of France, somewhere in Germany and somewhere in Sweden.

Addition 1: It seems there are actually two ferries involved in going from Sweden to Finland. So it might be a little more complicated and expensive.

Addition 2: The drive through Denmark and Sweden doesn't look longer than what would be possible without a layover, but then you should check when the ferries go (do they go every hour all night?) and possibly time your journey. Also it might not be all that fun to arrive at your destination in Finland in the middle of the night and then having to sleep away the first day you'll actually be in Finland.

Addition 3: All the mentioned countries in both mine and Willekes answer are Schengen countries so at least you shouldn't have to worry about visas, but if you decide on the route through Poland, Lithuania, Latvia and Estonia, and choose to deviate from the major roads be aware that the border between Poland and Lithuania ain't that long (just over 100 km) and if you go too far east you'll hit Belarus and if you go to far west you'll hit Russia.

I took a look at google maps and used Porto to Helsinki, which are 45 hours of driving apart, plus delays and stops. It gives the suggestion of driving through Spain, France, Germany, Poland, Lituania, Latvia and Estonia before taking the ferry to Finland.

Having heard stories about road conditions in that route from Poland north, I looked for an alternative.
If you are willing to spend more on ferries, you can drive a more northern route, going through the same countries till Germany but from there you take Denmark, Sweden and a ferry to Finland.
This route has a ferry where you cross from Germany into Denmark and one from Sweden to Finland.

You can of course take several different routes through the countries mentioned, or take roads that take you through other countries. Most people I know tend to plan the routes such that they avoid countries where you need a vignet (yearly toll for the motorways) and where toll roads are scarce or non existend, as such doing as little as possible in France, avoid Switzerland and do more in Germany instead.

I would suggest stopping for a good night of sleep at least after 16 hours of driving, if driving with two people, or after at most 12 hours if having to drive alone. Even if you are experienced in driving long distances and have done a few 16 hour ones, you will find that 45 hours in a few days are going to be much more tiring.
On such a long travel I would not book ahead but rely on what is available when you are traveling. If you travel on a popular date, use internet and book your motel while driving towards it, allowing you a much wider choice than when you have to stop at each possible one.

With such a long journey and the ferry (or ferries) near the end of them, I would not book ahead for them and just take the first available place on the ferry when you arrive.

But if you have to be in Finland for a given date, you may well plan your journey from that, allowing more time than you need and plan spend the last day before your ferry booking sight seeing near the departure port, which will allow for unplanned delays.