The FN-SLP is a gas operated shotgun....
but the Benelli M-4 is also gas operated....
The Benelli is a little bit lighter than the FN-SLP ( and weight is a big factor in helping to absorb recoil )...but I would think the felt recoil from each of these 2 guns would be very similar with the same shell. I also think either the M-4 Benelli or the FN-SLP would give you less recoil than any of the Benelli Inertia guns ( the M-2's ) even with the comfort tech system in them - but in my experience with the comfort tech system in Benelli's sporting guns ..the system works very well.
Which gun is faster...I don't know for sure / but I don't think I would say the FN is definitely faster. There may be no discernable difference in them in term of cycling speeds.
I have not fired the FN-SLP / but I have put several boxes thru the M-4 and its an impressive weapon - and a few shells thru M-2 versions of the Benelli as well ....and both the M-4 and the M-2's cycled very quickly.