If a commissioner has the time to sim 5 days a week or more, more power to him. There are plenty of commissioners that have plenty of higher priority things to do in their lives than run a baseball sim almost everynight. It just isn't realistic for many people. I am able to devote 3 nights out of my week to run sims and by running 7 day sims, the league can get through a season in a reasonable amount of time.
One thing you have to consider is leagues that go slow lose owners because if owners of teams that are performing poorly can't do anything about it via free agency and the amateur draft until a season ends 4-6 months later, then they lose interest. But if a league is going to complete 3-4 seasons per calendar year, then there is hope for those owners that they can turn their fortunes around within a reasonable time.
As for trading, from the perspective of a commissioner who has to process all of the trades, I too have often considered a trading cap, especially in modern leagues. Many times I have been in modern leagues as an owner and seen literally hundreds of trades done in those leagues prior to the first sim without any review only to see owners drop out when they've destroyed their teams via trade while others have become juggernauts. One time when I mentioned considering a trade cap, I almost had the entire ownership group leave the league. Trade caps are not well accepted

So, I have always implemented a trade commission in my leagues to review trades for fairness and balance so that the league doesn't get thrown off-balance with really bad trades resulting in teams that are so bad no one would want to take them over after the owner leaves. A trade commission helps to slow down the process a bit as well as owners know that they can't just go trading players they just got in one trade in another immediately. I set a deadline for owners to post and confirm trades, then I review them myself and if I feel a trade is questionable, then I pass it on to the trade commission. They're decision is final (I'm not involved in the decision) as to whether the trade is approved or not. Owners have the ability to contest the decision of rejection but providing a reasoned explanation of why the trade was executed between the two teams. Rejection of trades are rare when owners know that trades are reviewed for fairness. And this does have some impact in lowering the number of trades that can happen in a league. In reality, few trades are done in MLB, so why should our leagues get bombarded with an unreasonable number of trades?
Oh, and I don't let teams "officially" trade until the league is full. I'm sure owners talk about trades regardless but they can't post trade agreements and have them reviewed until the league is full. I don't think it is fair to allow owners to trade while there are still ownerless teams, but that is just my opinion.
Just my $.02
Dan