Spamassassin is slow - any tips or good commercial alternative?

[Charlie]

By the way, my CPU is Pentium 4, 2.4GHz and there is 1GB of RAM.
Emails are now taking 2-5 seconds to scan - is this normal for this 
configuration? We've disabled spam checking on emails larger than 40KB and 
that helped a great deal.

Server's details:

# free
             total       used       free     shared    buffers     cached
Mem:       1028576     835056     193520          0     117124     264136
-/+ buffers/cache:     453796     574780
Swap:       996020      21148     974872

# uptime
 03:34:13 up 222 days, 45 min,  2 users,  load average: 1.12, 0.71, 0.43

# cat /proc/meminfo
MemTotal:      1028576 kB
MemFree:        182964 kB
Buffers:        116612 kB
Cached:         263300 kB
SwapCached:       3028 kB
Active:         624508 kB
Inactive:       175076 kB
HighTotal:      122856 kB
HighFree:          236 kB
LowTotal:       905720 kB
LowFree:        182728 kB
SwapTotal:      996020 kB
SwapFree:       974872 kB
Dirty:             364 kB
Writeback:           0 kB
AnonPages:      419628 kB
Mapped:          25792 kB
Slab:            38296 kB
PageTables:       2372 kB
NFS_Unstable:        0 kB
Bounce:              0 kB
CommitLimit:   1510308 kB
Committed_AS:   815640 kB
VmallocTotal:   114680 kB
VmallocUsed:      2676 kB
VmallocChunk:   111648 kB

# cat /proc/cpuinfo
processor       : 0
vendor_id       : GenuineIntel
cpu family      : 15
model           : 2
model name      : Intel(R) Pentium(R) 4 CPU 2.40GHz
stepping        : 7
cpu MHz         : 2405.624
cache size      : 512 KB
fdiv_bug        : no
hlt_bug         : no
f00f_bug        : no
coma_bug        : no
fpu             : yes
fpu_exception   : yes
cpuid level     : 2
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca 
cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe up cid
bogomips        : 4815.03