Nintendo's Super NES is considered one of the best gaming consoles in history, but its Achilles' heel was that it was underpowered compared to its competitor, the Sega Genesis. While the graphics processor and sound board were powerful, the CPU in the SNES clocked in at a paltry 3.58 MHz, as compared to the 7.6 MHz CPU in the Genesis. The end result was that games like Mega Man would slow down and drop considerable frames on the SNES whenever significant action took place.
SNES slowness — Some early releases for the SNES didn't even take advantage of the processing power it did have, because memory chips that could read at faster rates were costly. The SNES would underclock its processor to accommodate games with "SlowROM" memory.
On software emulators this doesn't matter, so a developer from Brazil, Vitor Vilela, has decided to speed up these games by releasing patches that make the games running in emulation act as if they have access to the full potential of the SNES. Playing Castlevania IV with the "FastROM" patch, there's no stuttering and more enemies are able to render on-screen. You can play the game with the patch on stock hardware if you have a cartridge to write it onto.
Cheap fixes — Newer games for the SNES took advantage of the console's ability to interface with expandable chips that added even more processing power, which Nintendo felt was cheaper than adding an expensive CPU to the console itself. These co-processors, like the SuperFX chip, were built directly into game cartridges. The SNES had extra pins in its cartridge slot that were designed specifically with these chips in mind.
Vilela believes that his patch to older titles will allow them to take advantage of these expandable chips on stock SNES consoles, but doing so will require considerable more work. For many games, a simple boost to 3.58 MHz will be sufficient.
Vilela's patch to Castlevania took over 70 hours to complete, and if you'd like to support him, he has a Patreon where you can contribute and get access to new patch releases before anyone else.