I looked it up again and it seems you guys are right that it's 100% increase in time. I honestly did not feel that it was such a big problem while I was playing as Manchu.
A few things can seriously offset the increased reinforcement time.
1. Fight defensively at the beginning. Engage invading enemies' straggler armies whenever you can. Have some forts in mountains so that you can easily defeat besieging stacks. (This whole tip is to minimize casualties so the reinforcement time isn't as devastating.)
2. Lots of people probably don't realize this, but your reinforcement rate (RR) is actually 100% while your army is in your own provinces, but RR becomes 50% in neutral or hostile territories. (Think only 10% in uncolonized)
3. Hire the Quartermaster as advisor! +33% reinforcement rate.
4. Defensive idea group also has an idea that give +33% reinforcement rate.
I think the problem really is rooted in completely different attitudes toward war tactics. I suspect that a lot of new-ish players just start the war carpet sieging their enemies instead of letting them come to you first, and they probably also engage enemy stacks in unfavorable terrains across rivers. Tip 1 and tip 2 are basically how you're supposed to be fighting any war as any country anyways, but judging by videos I've seen, not many people actually do this. Only the most seasoned players religiously practice this defensive tactic.
PS: Think you can even skip #3 because the +5% discipline advisor is definitely better, but you don't always see him/her around. The moral advisor is also good, especially at the beginning of the game.
