Only my tub fauce water pressure starts great, then after 5 seconds it drops off.
It happens on both hot and cold settings.
What could be causing this?
Thanks in advance for any advice.
I think you've got debris in the lines & the tub is the perfect place to flush it all out. You'll want to turn on the closest faucet, with the aerator removed, to the Main Water Shut-off for the whole house & then turn off The Main.
You'll want to leave that closest faucet open for now & turn The Main on & off several times to make sure it itself is scraped clean & only clean water is coming out of that faucet. It's common for The Main to accumulate mineral build-up. Then, turn off that faucet, reinstall its aerator & turn The Main off.
Now, you'll need to go to the tub & remove those handles & their valves, if you have 2 of them. If you have a single handle, then that handle, its wall-plate & its cartridge need to come out. Then, shut the tub drain & door or curtain & go turn on The Main to flush everything into the tub.
That should clear everything & get you back to normal. If you're unfamiliar or uncomfortable then take pictures as you go or call a plumber to do this operation.
After you try Iggy's answer to clear the pipes, if you still have a problem, I would suspect the pressure balancing valve is malfunctioning. That is why only the tub is having this problem.
You would then have to disassemble the shower/tub valve and at least clean it. You may need to replace parts of it.