There is an official emulated version available on the PS3 PSN store. It was developed by a supremely talented small Japanese company called M2. In the game, they included an option to overclock the CPU and it makes a big difference. My favourite way to play the game.

M2 was also responsible for the 3D Sega classics on 3DS which are incredible.

https://en.wikipedia.org/wiki/M2_(game_developer)

On the Nintendo Switch there are plenty of Neo-Geo/SNK titles in the eShop, including Metal Slug, Samurai Showdown, Fatal Fury and others.

Most Neo Geo romsets will have it.

If not, just google for "metal slug 2 turbo" and you should find it on the first page.

But if you're playing emulated, you can just overclock the emulated Neo Geo and play without slowdown, right?

Probably. I play on a consolized MVS system so overclocking is a bit more involved and tends to cause issues in some games.

I don't recall any slowdown in Metal Slug 2 playing the rom in MAME in the early 2000s so you are probably right.