I honestly think it's more of a issue with the input delay than the lack of slowdown, but metal slug 2 on the ps2 anthology with none of the slowdown is really fucking hard. If the slowdown of 2 was somehow removed but the enemy patterns and bullet speeds were kept the same, the game might end up pretty unbalanced.
For example when you fight the stage 3 boss in ms2 on the anthology using the slug flyer, the boss fires a vulcan that leaves small gaps for you to navigate through, this is pretty easy to do on the MVS version, but I actually found it extremely hard in the ps2 port, the gaps are decently large but when they come at you super fast and your hitbox is gigantic it pretty much means a dead slug flyer for me, in real slug 2 that boss is cake for me.