They should all flash at the same speed. Might check all of the bulbs for issues and correctness, for good measure. If some of the bulbs might be LED replacements, without the resistance adapter, they can "hyper-flash", too.
If you have replaced your flasher check your grounds at the sockets in the rear and the ground wires at the front turn signal assemblies. Next check all your connections for corrosion, don't forget your firewall connection as its problematic.