Why did early rifled muskets take longer to load than smoothbore muskets?

Early rifled muzzle-loaders were slower to load than smoothbore muskets because of the tight fit required between the projectile and the barrel. To achieve the accuracy benefits of rifling, the lead ball had to be driven into the barrel’s spiral grooves, which made the process of ramming the projectile down the bore significantly more difficult and time-consuming.12

In contrast, smoothbore muskets used a loosely fitting ball that could be quickly rammed from the muzzle to the breech. While early attempts to improve rifle loading included using greased cloth or leather patches, the rate of fire remained lower than that of smoothbore weapons. This loading challenge was eventually addressed by the invention of the Minié ball, which featured a hollow base that expanded upon firing to engage the rifling, allowing the projectile to be loaded easily into the barrel without needing a tight initial fit.213