What tweaks? Link me please.
Head over to MAME Dev and have a read of thats been done to mame lately.
http://mamedev.org/releases/whatsnew_0120.txt 
Aaron (holder of the slowest mame driver crown) has been working his butt off, making all sorts of changes to some of the slowest drivers in MAME, including adding multi-threading support, 64bit compatiblity and other standard optimizations. That not to say the other developers haven't been doing great work just Aaron's work sticks out the most right now.
It's a myth that all CHD games are slow, it's the hardware most of them ran on that's the problem not their size, many ran 3DFX graphics, or multiply CPUs, or evil little CPUs like the R5000.
MAME ran into a preformance wall with many of these games for a while there

, MAME has now pushed though that wall with the release of the Core 2 Duo, and what's left of the wall has been falling to monster overclocking E6x50 G0 stepping Core 2 Duo's, 64 bit Windows, and now multi-threading.
Does anyone know which games are playable and actually worth it to download?
So to answer you question, Yes and No. No if you PC is running a Athlon or a Pentium (anything) they just don't cut it when it comes to emulating the really tuff stuff, on the other hand if you have a Core 2 Duo or even better an overclocked Core 2 Duo (these CPUs beg to be overclocked) then many of these games are playable to very playable depending how hard you make your CPU sweat.
So don't listen to those who say it will take a 10ghz cpu

Get thee to a computer store, pick up anything in the Core 2 Duo E6x50 range, something to give it a case of the chills @ stock clockspeeds, get a good P35 chipset motherboard, and a couple of GB of DDR2-800 or better ram, Overclock that CPU to within an inch of it's life

, install Win XP x64 or Vista 64 grab a copy of the latest optimized 64bit MAME build enable Multi-threading and have some fun.