Memory performance
To highlight the biggest difference between the Pentium III and its younger competitors, we'll start off with memory bandwidth tests. (Well, we pretty much always start off with those, but that's my excuse this time 'round.) The modified Stream test in SiSoft's Sandra does a nice job showing the peak bandwidth potential of each of these platforms. Here's how they stack up.

As you can see, the Pentium III is at a distinct disadvantage here. It delivers about half of the memory bandwidth of the Athlon DDR, and about a fifth of what the Pentium 4's dual Rambus channels manage to crank out. Now memory bandwidth isn't everything. In fact, its impact on real-world performance isn't huge—the Athlon gained between five to twenty percent with the move to DDR memory, and generally closer to five. Also, bandwidth is only one measure of memory performance. Memory latency is probably even more important to real-world performance (though the two things are related). Although harder to measure than bandwidth, the PIII's SDRAM probably has lower latencies than our other two contenders—especially the P4's RDRAM.

Memory performance also interrelates with L1 and L2 cache performance. Let's look at this another way, so we can pull all of those elements together. The snazzy-looking Linpack graph will do that neatly.

This graph's not always easy to read. For simplicity's sake, I've omitted the Pentium 4 1.6GHz from this one. For those of you who don't know how to read a Linpack graph, I've shamelessly stolen my explanation from a previous review and put it in here:

Linpack performs floating-point operations on a range of data matrices, and the resulting line graph shows the strengths and weaknesses of each processor. The Athlon shows its floating-point prowess by offering the highest peak performance with a relatively small data set. But once we reach about 192K, the Pentium 4 has a pronounced lead. Its 256-bit data path to its L2 cache, combined with a very smart L2 cache controller, helps put the P4 on top. Note that the Pentium III, which also has a 256-bit L2 cache interface, has a similarly shaped curve, and peaks at about the same place as the P4.

Both Intel processors start to drop off sharply at about 256K, while the Athlon hangs on until it reaches about 320K. Here you can see the Athlon's exclusive L2 and L1 caches working together. Because the Athlon's L2 cache doesn't replicate the contents of its 64K L1 data cache, its total effective cache size is larger than either of the Intel processors. (The Athlon also has a 64K L1 instruction cache.)

Once we get to those sharp, downward curves, we're accessing main memory to perform the calculations. And once that happens, the Pentium 4's fast front-side bus and dual RDRAM channels kick into high gear. The Pentium 4 delivers well over twice the sustained performance of the DDR SDRAM-based Athlon system with larger data sets, and it crushes the Pentium III, as well. A very impressive showing.

The Pentium III's L2 cache is very fast. At about 192K, it peaks out faster than the L2 caches on both the 1.4GHz Pentium 4 and the 1.2GHz Athlon. However, once we're out to main memory, we see the same pattern the Stream tests showed us above: the PIII is much slower going to main memory than its competitors.

I should also mention that our PIII test system was crippled by the fact that I'm cheap. (If you don't believe me, check this out. I'm rather sad, really.) We used low-brow generic CAS 3 memory instead of widely available, and faster, CAS 2 RAM, because that's what I had on hand. Some of the newer PC133 SDRAM DIMMs I've bought recently might have delivered better performance, but they were high-density chips, and the 815 chipset doesn't take kindly to them. It's also likely this Intel motherboard uses rather conservative timings, sacrificing some performance for stability. We wouldn't be shocked to see memory bandwidth scores another 100-150MB/s faster for a properly tuned Pentium III system.

That said, memory bandwidth will have an uneven, and sometimes very minor, impact on overall performance.