This is gonna be a double thread addressing the invincibility bug and performance issues on PC.
Invincibility
The invincibility bug is where all players on the server become invincible to sword strikes and bullet hits for a period of time. It seems like it is only an issue affecting galleon arena. It can last for only a minute or it can last the entire game. The bug has been in the game since August. The only way to inflict damage to players during the bug is by hitting them with cannon balls or now using the newly introduced fire to burn them. I've had games where invincibility lasts the whole match and I've also had games where it goes away and then comes back. I've posted two YouTube videos at the bottom of this post showing the bug in action.
PC Performance
Performance on PC has seemed to have gotten worse with the introduction of the new update. I have experienced many substantial FPS drops of around 40-50 depending on what I look towards in arena. Since I know people will ask my GPU is an RTX 2070 and my CPU is an Intel i7-9700k. I play on rare graphic settings. My friend playing on an RTX 2080 and Intel i9-9900k has noticed performance issues as well and he plays on cursed settings. I have only noticed it in arena especially when I look at islands but also on the open sea. I only get 50-65% usage on my GPU in game. Another issue I want to address regarding performance is the main menu screen. I'm talking about the screen where you select your ship and invite crew members to set sail. My GPU usage and my friend's GPU usage goes up to 90%. My fans speed up because of this as my PC gets hotter. I don't think the menu screen should take more effort from my PC than the actual gameplay.
As stated above here are the two videos showing the invincibility bug.