Silver on most events - 105% of the final time - for most people (the middle of the bell curve) should take one lap. That's 1m for ~100s. Even for those who can’t make it to silver on lap one, the +110% bronze can be done entirely on Autodrive (full braking and steering assist) on lap one, for 250,000cr in the same time.
Thus even the most inept players (and those for whom physical mobility and ability is an issue) can get 2,500cr per second from a Time Trial. At the equivalent of nine million credits an hour (36m for drivers of moderate ability, 72m for the talented who can one-lap gold it) it is objectively the most efficient way to earn credits in the game.
It drops the more time you put in, if you want a better grade of course. I've never spent more than 40 minutes on a TT - Monza in the Genesis VGT, when I was irritatingly close to safe gold but couldn't get my optimal sectors in the same lap - so even that was 3m credits an hour.
I've found some silver times to be really difficult to get on the TT's. I have mostly silvers, about 4 or 5 golds and 4 or 5 bronze medals. But like you, if I spend more than 20-30 minutes on a TT, I'm wasting my time.
Now, I've already stated that I'm not a fast driver. I'm a C rated sport driver but I do play the single player game at Hard difficulty and don't find it overly difficult. There are a handful of races that are actually really difficult, but the majority of the weekly challenge races are simple to win in the suggested PP on hard difficulty. I also know from reading the Weekly Challenge thread that there are a lot of people who play at much lower difficulty levels and can still struggle in the weekly races.
So, to say you can get 250k credits in one lap on the weekly TT's is not really true. There may be some tracks where it's easy (ie. Daytona oval), but I've found that I can't get bronze on my very first try most of the time. It's usually a track limit issue or a tap to a wall or just crappy tires that have no grip and spinning out. It can still take about 3 or 4 laps (approximately 10 minutes) to get used to the car and the track, then another 10-20 really trying to push myself. Someone like me will usually land in the silver range, but what about those people who have to play the single player game at easy difficulty? Do you think they are getting silver on the TT in 30 minutes? Likely no (barring the easy ones like the Daytona Oval). So 250k for 20-30 minutes of driving isn't really that efficient to me.
Like you said, if you are a naturally fast driver, or are just a fast driver from practicing a lot, then golding a weekly TT in 5-10 minutes is actually a very efficient way of making credits in the game. Get the gold and move on with your life. Unfortunately, the majority of the drivers who can gold a TT in a matter of 5 minutes aren't chasing credits to buy every car in the game. They are usually spending the majority of their time in sport mode grinding the same 2 tracks all week long in the meta car for the week. So no matter what we do in this game, it's a grind.
Moral of the story, find your favorite grind and just drive. The credits will come naturally that way.
