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Compressed games run slower because your CPU has to decompress data on-the-fly. If you experience lag, use the .iso file instead (larger, but faster).

(~400MB) : Surprisingly, the PS2 port of this PC classic is very efficient. It even includes the "Decay" co-op expansion, making it a high-value download for its size. Metal Slug Anthology

If a game is "ripped" to under 500MB, you will likely lose the cinematic cutscenes. The game may skip directly from gameplay to gameplay, leaving gaps in the plot.

The Mechanics and Culture of Highly Compressed PlayStation 2 Games: A Technical Analysis of "Under 500MB" Archival

Highly compressed PS2 games under 500MB are often "ripped" versions of original titles where non-essential data—such as high-quality audio, cinematic cutscenes, or "dummy" padding files—has been removed or downsampled to reduce the file size. While the average PS2 game originally ranges from 4.7 GB to 8.5 GB, these modifications allow users with limited storage or slow internet to download and play them via emulators like PCSX2 or AetherSX2 . Popular Highly Compressed PS2 Games

We can look into a on how to convert your own large PS2 ISOs into the highly efficient CHD format .

If you are looking to save storage space while playing classic titles, many high-quality games come in surprisingly small file sizes or can be highly compressed using formats like 7zip (.7z) or GZ .

This practice also raises significant legal and ethical questions. While emulation itself is legal, distributing compressed ROMs—even heavily modified ones—constitutes copyright infringement. It deprives rightsholders of potential revenue from re-releases (such as the PS2 Classics on PS4/PS5) and undermines legitimate preservation efforts by organizations like the Video Game History Foundation, who argue that maintaining bit-perfect copies is essential to preserving the experience —including the grain of a 2002 MPEG-2 cutscene or the loading-screen CD audio crackle.