First of all, just understand the rarity of a 6-3, nearly 227lb man running a 4.44 40 yard dash.
The closest size/speed comparison you can find is Lamar Jackson at 6'2, 216, who lead the NFL in QB rushing yards last season. Funny thing is Jackson never ran at his pro day or the combine, and there is no official timed 40 on record, which is a little suspicious. University of Louisville claims he ran a 4.34 unofficial, but that is it. No doubt Jackson can fly, but as he was considered a fringe prospect going into the draft if he could really run a sub 4.4 consistently its likley he would have done so to improve his draft stock.
The next closest comparison, and by speed only is Kyler Murray. Murray ran an official timed 4.38, a year before he was drafted, at 5-10 and at the time he weighed less than 190 lbs. Murray can fly, no doubt, and that speed is official. But he was 5 inches shorter and 40 lbs lighter than Fields when he ran it.
In the heavyweight class, you could compare him to Cam Newton, who does top Fields at 6"5, 248 and ran a sporty 4.59, which is just amazing for a man that size. Cam was 20lbs heavier and was more of a bruiser, but for Fields to absolutely smoke him in a 40 at only 20 lbs lighter just shows the elite athlete that Fields is.
The only negative comparison I could find was RG3, who at 6-2, 220 ran a 4.42. He didn't have the NFL career that was expected of him but injuries played a huge part in that as well as his inability to develop as a pocket passer.
Bottom line, Fields is a generational athlete at that size and speed and only a few players in the history of the NFL come close to that athletic ability.
