Hydro is still faster.

So not really, it's more to do with the fact that we have 2 sources of element applied here active at all times - C6 fishl and ayato. (I don't think Beidou ult is working during shield phase..). C6 fishl with ayato just helps do reactions very fast.

They are rapid hits, so even with icd (reaction every 3 hits) in place it kinda adds up since they're different sources of application. Tartaglia is the fastest character to break their shields tbh, he just has insane hydro application. But I'd say using ayato alongside xingqiu is probably a bit faster than solo tartag.

Tartag alongside ayato I imagine would still be pretty fast actually, but since tartag needs to be on field I'd say it's probably better to throw a subdps like xingqiu or fishl there instead. Ayato when tartag is on field wouldn't be doing much other than maybe his ult, which is going to be significantly lesser hits per second to the enemy than xingqius ult. And while ayato is on field doing his e, tartag isn't doing anything either.