Tub faucet pressure starts strong then drops off
I think you've got debris in the lines &amp; 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 &amp; then turn off The Main.

You'll want to leave that closest faucet open for now &amp; turn The Main on &amp; off several times to make sure it itself is scraped clean &amp; 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 &amp; turn The Main off.

Now, you'll need to go to the tub &amp; remove those handles &amp; their valves, if you have 2 of them. If you have a single handle, then that handle, its wall-plate &amp; its cartridge need to come out. Then, shut the tub drain &amp; door or curtain &amp; go turn on The Main to flush everything into the tub.

That should clear everything &amp; 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.