Startup USB pianfully slow, will External USB HD be faster?

The size of the drive will not make it boot faster. If you had a FW drive it would be faster but to make a backup just let it run overnight.

Are you making a clone or a Time Machine backup? I suggest you create a clone. It's faster and will better serve your needs than Time Machine. A clone is bootable but Time Machine is not.

Both of these applications can be used to create a clone.

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USB 1.1 = 12 Mbit/s

Firefire 400 = 400 Mbit/s

USB 2.0 = 480 Mbit/s

FireWire 800 = 800 Mbit/s

USB 3.0 = 5 Gbit/s

Thunderbolt = 10 Gbit/s

USB 3.0 is 10x faster than USB 2.0

Thunderbolt is 12 times faster than FireWire 800 and up to 20 times faster than USB 2.0.

A USB Flash Drive is NOT a fast device. A USB hard disk will be faster than a Flash device. You internal disk will be faster than the USB hard disk, only because it has a faster interface, but the USB hard disk will beat just about any USB Flash device.

I have bootable USB flash devices for emergencies, but not to run my system normally.

I created a startup disk using a 64G USB but it is painfully slow. Would the Mac run faster if the OS was on an external USB 3T HD?

If the 64Gig USB that you refer to is a thumb drive, yes, it will be slow. They are not in the same performance league as hard drives.