I posted this on reddit, just copying here as it is basically a guide for speedrunning the game. My friend and I beat it in 44 days, just before the winter (which would probably slow down every run). Here are some important things we did/learned:
Early game greed waves: So what surprised us the most when we started attempting this was the ridiculous easiness of the greed attacks. Really, the normal nightly attacks are nothing.
No walls and towers: Since these attacks were so easy, improved walls and any towers were not necessary at all. The money needs to go to the boat, archers and upgrades. The basic and further improved walls you get for free with the upgrades. The only wall you will want to make is when you need to get space for the forge to purchase the bomb.
Don't destroy portals: Really, only time you should spend money and time on attacking is when you have the bomb. Go out and destroy each small portal after the other while escorting the bomb. Two weeks ago I asked here whether one could just ignore the portals, but that's not the case. However, since it's so early in the game, the counterattack waves are only slightly stronger than the usual ones meaning that you can happily continue escorting the troops as long as you have a combat mount. This also means that you don't destroy any portals on the seaside and therefore don't create a lighthouse on any island. We haven't once encountered a big boi during the run and were very surprised to encounter three floaters at the very last counterattack of the entire game. They luckily didn't ruin the run.
Only get the most important upgrades: So, obviously the stone and iron are something you need. Other than that, we got the builder statue and the archer statue for stronger walls and more accurate archers.
Mounts: On island two, I got the stag to get more speed during exploration and more income from deer. My friend took the gryffin once we went back to island one. On island four when we did our first attack I switched to the lizard for some extra fire power. Then I switched to the unicorn as soon as possible on island five to get crazy income. In a normal run, I don't think the unicorn is worth that much as you are earning so much money anyway, but it was sooo good to have here. I switched to the lizard again to attack and back to the unicorn after. This switching continued on the remaining islands as we decided to get stables.
Island order (1 - 2 - 3 - 1 - 4 - 5 - 2 - 1 - 3): First bit is logical, just pass these island quickly, then we went back to island 1 for the statue and gryffin. The boat is super cheap anyway so you can be on your way after one or two days. After that you only need to go to four to get iron and we could start destroying the cliffsides. After 5 we went to island 2 for the stable hermit as we were making this strategy as we went along (the unicorn wasn't planned), so you could get the stable hermit on the first visit to island 2. Finish at island 3 or 5 since then you don't need to buy the boat again. We did 5 earlier in the game since 5 will probably be harder to beat. Also the unicorn thing.
Alternative routes might be leaving 5 for last, or going to 1 instead of 2 after 5/4.
Chop trees: When we were nearing the end of each island, we started chopping down every tree there was for that small bit of profit. The fields for bunnies are also nice of course, but by then you'd probably already have those fields up and running.
