i kinda wanna use 1 phase blade + 1 CS grief

but will my WW speed be slower than using 2 phase blade?

Yeah it will look at your fpa for each weapon and average the 2.

Check out the ias calc:
https://warren1001.github.io/IAS_Calculator/

Check the weapon attack speed calculator. I'm pretty sure you should get to max fpa, not sure If you need extra IAS on gear and how getting slowed would effect the whole thing though.

For pvm, there is no reason not to use phase blade

i swapped merc to a faith merc, now i get max ias

Well that’s a terrible idea

Why would that be a terrible Idea? I did the Same.

you can get max speed with phase blade

Then you can use a better merc... gasp...