If you're curious how fast Justin Fields is take a look at the end of this highlight video below at just pass 6 min mark - a subtle but nice highlight, Fields starts back about 7 or 8 yards and easily overtakes his own RB and Spartans FS in 30+ yards or so to provide some support blocking.
In the NFL, he would probably land at 3rd or 4th behind Jackson and Murray, and level with Hill, and a bit faster than Mariota.
That speed will translate well in the NFL, but the knock on Fields (like many in the NFL) is his reads. NFL QBs must go through their progressions in under 3s and optimally 2s with today's pass rushes.
I've seen some mock draft with 49ers trading up for Trey Lance - I think that'd be a mistake. Lance looks decent but not worth extra #1 picks.. I think they're targeting Fields or Wilson.
I actually don't much like the trade-up - its a fairly big gamble trading away 2 additional future top players for QB now. Some may see It as necessary, and it may or may not work out, but its a bunch to pay - 2 additional #1s and #3. Like any gambling, the more wagered, the bigger the win or loss. If it doesn't work well (ie Wilson or Fields flop), the 49ers would be giving up possibly top 10 draft picks in subsequent years - enough to get top positional player twice. Even the fastest players at QB get flummoxed in the NFL.....the speed comes in handy, but if a QB makes too many bad decisions, it won't save a loss. One thing for sure, people will have higher expectations.
On a similar type note, this next year it'll be interesting with the Saints - see how good Taysom Hill is as the principle starter with this throwing over the whole year.