In November of last year, the Xbox One added backward compatibility, bringing a treasure trove of great Xbox 360 games to the newer console. While effectively emulating the older 360 is a technical achievement to be sure, some games have been found to suffer from massive performance issues.

With more than 100 older games now playable on the Xbox One, Eurogamer’s Digital Foundry took on the task of testing every major 360 game that can be played on the console. Many games perform as well or better than they did on their home console, but some of the most popular games have been found to be “nearly unplayable.”

One backward compatible game that many Xbox One owners will likely hope to play is Halo: Reach, since it’s the only Halo game not already playable via Halo: The Master Chief Collection. Unfortunately, the framerate often dips as low as 15 frames per seconds, and even when it manages 30 frames per second, Digital Foundry says inconsistent frame persistence keeps the game from ever feeling smooth.

Gears of War: Judgment was the final Gears of War game developed for the 360, and like Halo: Reach, it has trouble maintaining a consistent framerate on the Xbox One. Most battles in the game – and keep in mind that this game is mostly battles – see the framerate dropping to 20 frames per second. Even when no enemies are on screen, larger environments drive performance down.

Fortunately, the rest of the Gears of War games were found to perform well. While Xbox One owners will likely opt for the remastered Gears of War: Ultimate Edition, the original Gears of War performs well via backward compatibility, often with better performance than on the original console. Gears of War 2 is marred by occasional slowdown, but generally plays as well as it did on the Xbox 360.

Digital Foundry notes that, in general, backward compatibility has improved since it originally tested many of the games during the beta. For more information on other games and why Xbox 360 emulation is so tricky, see the full rundown at Eurogamer.