I've never played lacrosse or American but I'd be surprised if there weren't big hits in both.
I don't know much about either but I'd expect the harder hits to be in NFL. Thats because there's more money in NFL so I expect that the biggest strongest guys drift to NFL and there is a wider base of players to choose from not because footballs a naturally tougher sport. I'm guessing the impact speed are roughly the same and that Lax padding is gives the same sort of protection as football padding.
You can't compare two sports without comparing the level at which the sport is played. I'd expect Pro lacrosse players (are there pro lax players?) to put in much bigger hits than high school football players. Example, played rugby from the age of 4 to 18 without injury. Went to University and played a much higher standard against much bigger, stronger and faster people. Result; surgery at the end of every season for 4 year straight and still needed another one latter.
There was a TV show called Sports Science on in the UK that did a special on the hardest hits. The test methods they used were pretty crude but it was interesting to watch. Results, impact in Sumo and Ice Hockey were much lower than the rest. This really surprised me as I've always thought that Ice Hockey looks very physical but I think the test method they used was pretty dodgy for Ice Hockey.
Next up was an NFL player blind siding a dummy square in the back. Can't remember the force generated but it wasn't that high compared to some other hits (but it was over 1000lbs which was more than they got from a sledgehammer).
Then Rampage Jackson's punch at 1800lbs.
Then was rugby. Some idiot somewhere decided that getting hit while receiving a hospital pass is the hardest hit in rugby. How this beat out getting tackled when receiving a high ball, getting dump tackled a bit side of head on or a head on tackle/collision between two centres/crash ball specialist (definitely the hardest hit legal hits I've received and given) is completely beyond me . I guess someone would just end up in hospital if they tried this. They also forgot the highest impact in the game, the engagement in the scrum. A 2000lb mass of freaks smashing into another 2000lb group of freaks with pretty much all the force going through the loose and tighthead props necks and upper spine. There's a reason why props get paralysed or killed on the pitch more than every other position put together.
Anyway, rant over; initial hit was 17000lbs and then the hit on contact with the ground was 20000lbs. They noted that the force from the impact was very highly contracted rather than spread out. They used real people for this one rather than an athlete and a dummy. But, they used American rugby players rather than Pros from Europe or the Southern Hemisphere. Anyway this was a load of craps as hitting as target that is prepared to take a hit is very different to hitting a dummy.
Next up was an NFL player hitting a dummy head on. The dummy was on a weighted sled which didn't move much so the dummy was basically fixed. Impact force; a lot, can't remember but considerably higher than rugby. But they measured the force on the suit not the force transmitted to the body underneath. Here's another NFL player who's supposed to be good though:
Finally...the Winner...Rampage Jackson with a body slam. Its looks like a spear tackle (rugby definition of spear tackle, i.e. very hard, very dangerous and a penalty offence at least for a very good reason) from a static start to me. But the results are huge, 400g of acceleration on the head. Thats HUGE. Admittedly it was using a dummy that could resist the tackle but I've seen enough spear tackles and resulting hospitalisations to know that this would fuck you up if executed well.
