It's because of some internal issues with valorant. One of the devs made a security change to how an executable worked and it had a drastic impact to fps that hasn't been fully resolved yet. He said it was reverted and fps should return to normal, but clearly that's not the case because before I was getting 240-350avg and now I barely stay above 144.
The security change that was causing performance issues has been well and truly fixed... but there were several other changes that have slipped in causing performance to regress.
Right now we've increased the number of people working on performance issues and so we've got some improvements coming in the next patch. We're also identifying and planning out longer term work to improve performance, especially on high end computers.
It's hard to say for sure but I expect that performance after this next patch will be similar to the performance of the first beta version and then the patch after that should be overall better. (Your mileage may vary, this is just an estimate, etc, etc)