From my personal experience Pacu grow at a tremendous rate. I got one at half dollar size and in less than a year he was 16" and ate everything including air lines, air stones, thermometers, uplift tubes, plastic plants, etc!!!!
As for my slowest growing fish I think would be my convicts. Takes forever to fully grow out a con.
obviously stuff like airapaima and RTCs are gonna be the fastest, but of the fish i've owned my senegals have been the fastest growing, got them early october at about 2.5" and now after a little under 2 months they're pushing 5.5-6", slowest grower being my opaline gourami who has grown about 1.5" since july