Ubuntu and SuSE are the same kind of thing: an easy-to-use Linux. Yet they're somehow mystically faster? Fair enough. Let's see the system requirements as published for these distros.
Linspire:
PC with 800 MHz or higher processor**
128 MB of RAM (256 MB or higher recommended for best performance)**
Hard drive with 4 GB free space**
SVGA or higher resolution and monitor** (3-D graphics accelerator card for some games, screen savers, etc.)
CD-ROM or DVD drive, Keyboard & Mouse
Linspire-compatible sound card and speakers or headphones**
Linspire-compatible 56 Kbps hardware modem, cable modem, or DSL modem**
Ethernet card for Internet/LAN connectivity**
(Off topic, but the Linspire website is an utter ripoff of Apple's. Pathetic.)
Mandrake:
Processor: an x586-class or above processor is required. This includes Intel Pentium I/II/III/IV/Celeron, AMD K6/II/III, AMD Duron, AMD Athlon/XP/MP. Hyper-Threading is supported. SMP multi-processor machines are supported. (*)
Memory: at least 64 MB is required (32 MB for text-install); 128 MB or more is recommended.
Hard disks: IDE, SCSI and S-ATA hard disks are supported.
Hard disk size: At least 500MB is required, 1GB is recommended. Large capacity drives are supported (up to 250GB).
Fedora:
CPU Requirements
Minimum: Pentium-class
Recommended for text-mode: 200 MHz Pentium-class or better
Recommended for graphical: 400 MHz Pentium II or better
Hard Disk Space Requirements
Custom Installation (Minimal): 620MB
Server: 1.1GB
Personal Desktop: 2.3GB
Workstation: 3.0GB
Custom Installation (Everything): 6.9GB
Memory Requirements
Minimum for text-mode: 64MB
Minimum for graphical: 192MB
Recommended for graphical: 256MB
Those three aren't so good. Mandrake perhaps the best of them. But what about what I listed? Ubuntu lists no requirements, but I know it ran fine on a P2/400 with 128MB RAM.
SuSE:
Processor: Intel: Pentium 1-4; AMD: Duron, Athlon, Athlon XP, Athlon MP, Athlon 64 and Sempron
Main memory: At least 128 MB; 256 MB recommended
Hard disk: At least 500 MB (for minimal system); 2.5 GB recommended for standard system
Sound and graphics cards: Supports most modern sound and graphics cards
Now...nothing listed is a "hard" distro that supports old machines. Those are all the easy, "bloated" ones. I'll give you Fedora; it's slow. But I've never had performance problems with Mandrake, SuSE, or Ubuntu, on said P2/400s with 128MB RAM. (My school has a lot of those.) It's actually kinda gotten better: Ubuntu Warty had a laggy Gnome 2.8, where opening the Run Application box took upwards of 10 seconds. But Hoary changed this to about 2, and is more responsive overall. I've only worked with Mandrake/SuSE 9.2, so I can't say if Mdk 10.1 would be faster on such machines.
You're not limited to harder stuff for old machines. That is, provided you can get 128MB RAM for them. Which you likely can.