Some tips: since the bit will be doing a shearing cut there will be some fiber tear that you will need to sand out after flattening. The bigger diameter the bit the faster the cutting edge is traveling and the better the cut for a given feed speed. Get a fairly large diameter straight bit, 1.5"?? Since you will be cutting fairly shallow the length of the cutting edge doesn't need to be any longer than absolutely necessary. Shorter is better than longer. How fast you can move the router will depend on how much power it has and how deep your cut is. Hang on tightly as the bit may want to accelerate the router if you start climb cutting. Make sure your rails are sufficiently stiff so you end up with a flat cut. Shim any twist to average out the cut.
Many people use router sleds, slow but get you there. You can surface about any width of slab, not true of a jointer. I have a 16" jointer and a 5X10' CNC router. We have been surfacing big walnut slabs for a man who makes tables. Even with 12 hp & a 4" diameter cutter it is still not super fast when some of the slabs have a lot of twist... We also have a 36" widebelt sander to do the final sanding and take out the grain tear. It can take up to 40 minutes to surface & sand both sides of a big slab.