Hardware Upgrades -- CPU

Although NT 4 (and reportedly NT 5) will run on older processors (486s and under-100MHz Pentiums), the proliferation of video and multimedia both in everyday applications, and over the web, might prove a bit much for the older chips. And since speeding things up no longer requires a neither a motherboard transplant, nor a large fraction of your computer's resale value, it might be well worth your while to investigate a CPU upgrade.

Other useful upgrades for older systems might include L2 cache (if possible), faster hard disk drives, a newer video card, and, of course, memory. But if you're relatively happy with those things (you're not crying over them), and you're not running at least a 100MHz Pentium (see bottom of article!), your processor is probably next in line for an upgrade.

Determining a CPU Bottleneck

There are a few Performance Monitor counters that deal specifically with CPU performance measurements.

"CPU Usage" is both a PerfMon counter as well as a counter on the status bar area of the Task Manager. The "conventional wisdom" is that CPU Usage hovering around and above 80% constitutes a CPU bottleneck.
If the "Processor Queue Length" counter (object = System) is consistently reading around 2 or higher, this also indicates a processor bottleneck. (This counter measures the number of objects that are ready but waiting for the processor).

More Information

The Knowledge Base article Optimizing Windows NT for Performance further explains these, and other, CPU counters.

Also, if you register (free) at MSDN Library Online, you have access to the Windows NT 3.51 Resource Kit, including Volume 4 (How to Optimize Windows NT), and furthermore that volume's entire "Detecting Processor Bottlenecks" chapter.

While all the [many] pages of this chapter are framed (thus needing the main MSDN frame to "turn pages", thus the futility in providing links to particular chapters and sections), one section near the end of the chapter, Getting Rid of a Processor Bottleneck, is complete on it's own (no other "pages"), and very enlightening. Note that this is an older document, and alludes to "clock doublers", but it's still relevant.

CPU Upgrade Manufacturers

Both Intel and Evergreen Technologies offer upgrades for Pentium-class processors, and for some 486s. Intel used to offer clock doublers for older 486s, but have discontinued them. If you've got an ancient 486, and are willing to spend almost $400 to boost it to a "Pentium 133MHz" (reality is more like a Pentium 75MHz), you might check out VisionTek's ExtremeCPU upgrade for 486s.