This has been discussed in the past with well documented details, but in essence ..
Each version of MAME (or any project) changes .. adding more stuff, or changing old stuff; the MAME code has been substantially changed over time as its a fairly old project. Originally when it suported only a dozen games, its requirements internally were very different than when it supports stuff like Street Fighter Alpha, so the code changes. Rather than have dozens and dozens of very different code paths for different architectures (an unmaintainable mess), they simplify; probably the right thing to do in a lot of ways, but it does 'write off' older machines. ie: At very specific versions, they did changes that hurt less beefier machines -- consider, they used to support 8bpp paletted modes, and 16bpp, and 24bpp, and so on; at some point it was just too much burden to carry all that around, so they simplified and dropped 8bpp palette modes IIRC. Likewise for other changes .. at one point they supported dirty buffers, and now not. Stuff like that .. at specific versions, from version 1.2.3 to 1.2.4 (made up), the speed woudl drop 15% just like that.
So Pandora can run _current MAME_ reasonably well (its out there), and also an older mame like MAME4ALL much much more efficiently -- but with a featureset more or less locked in history. You get your pick.
jeffphone