Y halo thar
As a HL1 mapping/development community I thought you might be interested in a BETA build of ZHLT 3.4 that makes use of SSE2 instructions on processors that have them (Intel: Pentium 4 or later, AMD: FX lines or later and most 64-bit processors).
Amckern, who currently maintains ZHLT gave these to the Sven Co-op development team, and most of us have noticed roughly 15%-20% speed improvement on HLRAD after comparing compiles with and without the SSE2 builds.
These compilers WILL require a processor capable of SSE2 instructions, you can use CPU-Z to or something similar check if your processor has the SSE2 instruction set.
Downloads:
Mirrors for 32-bit (IA32) Windows: Amckern's, AdamR's
Mirrors for 64-bit (AMD64) Windows: Amckern's, AdamR's
Mirrors for all Windows compiles, contains latest non-SSE2 builds too: Amckern's, AdamR's
Feel free to mirror these - please ensure you tell users about the SSE2 requirement if you host them elsewhere, and the fact this is a BETA build.
About multi-threading / 64-bit:
The compilers DO multi-thread (and seem to do so very well), so dual/quad core processors could probably see a near double/quadruple performance increase. You can use the -threads X parameter to limit the compiler to a specific number of cores if you wish.
As for 64-bit, I've not done any benchmarking - however when I switched from XP Pro to XP Pro x64 I did notice a fair chunk of time difference, however I can't say for sure if it was half or anything that drastic.