Hey I have a similar problem, thought I'd post here before trying to make a new topic. My PC isn't old but I'm getting bad frame rate drops around multiple players; Warspear, Shat and Dal when timewalking vendors are up. Besides those situations my frame rate seems great.
My specs are - Win10 i5 4690 @ 3.5 8gb RAM gtx970
I have dual 1920x1080 monitors but only game on primary one, my temps are fine.
I've messed with a lot of settings and addons but I'm getting bogged to 45 fps at timewalking vendor. AA, shadows, vsync, nothing gave a big noticeable change.
Any suggestions for a specific setting or weak part of my PC? I think my CPU is weakest part, ram would be cheapest upgrade.