Where to begin?
First off, if you are going to be doing a lot of aluminum cutting, you really do want a mill, and will probably be disappointed in the long run with a router table. (You didn't mention cutting anything else but metal.) A mill will be much more rigid and have a spindle that is designed for the large sideways forces that a mill experiences. Routers simply don't come with bearings of the same class as mill spindles.
That being said, many DIY CNC'ers have successfully cut aluminum with their router table, me included, but it is a mistake to generalize about cutting speeds like that.
A router bit (or a mill bit) has a cutting edge (1 or more, the number of which dramatically affects the cutting speed) of HSS or carbide (which one also affects the cutting speed) against the material (aluminum of some indeterminate alloy and temper - which one affects the cutting speed) that moves a settable rate (the feed rate affects the cutting speed), the depth of cut (DOC) which also affects the cutting speed, as well as the speed that the cutter moves around its axis (the RPM of the router, which is often settable on some routers, but usually at the expense of power.)
For a given material and a given cutter edge type set at a given velocity of the cutting edge against the material, there is a "sweet spot" of feed rate, RPM, and DOC where the chips (or swarf) that is produced comes off with minimal force and minimal heat (no melting - if the chips start to melt you've probably ruined the piece and maybe the bit as well.)
Therefore several things affect the speeds - you can often set the RPM much higher simply by switching from a four flute (edged) mill to a single flute (edged) cutter, and use a shallower depth of cut all while maintaining the same feed rate. An aluminum "O" type single flute cutter will indeed cut aluminum at 18,000 RPM at a relatively shallow DOC.
There are tables of these sweet spots that you can look up spindle rpm and feed rate for a given material, but these generally assume milling operations, among other things so you have to be careful to stay within the bounds of what your machine can tolerate, as well as what your bits can do. It is very easy to snap off a 1/8 inch bit if your feed rate, DOC, and RPM's aren't set carefully, regardless of how many HP your router has.
In any event, unless your machine's manufacturer can provide this data, you need to keep a logbook of what the above variables are set at when you make a successful cut in any material. Start with a shallow DOC and work your way up. Eventually you have some good data to rely on to cut any material for any given machine. You can get a rough idea by hanging around other CNCZone forums where they've already been doing this for a while.
Just a suggestion,