Rank Math slowing site down so much I had to deactivate the plug in I am paying

After using Rank math PRO for some while my site slowed down so much it is not possible to work backend or frontend, when I deactivate rank math site works like a charm. I tried contacting their support but all that they did is reinstalled the plug in, after that it was ok for a few days and then again problems. Again I contacted their support, and again they asked for credentials but this time when I sent credentials there was a guy that did not know the difference between ftp and sftp, I sent sftp log in credentials but he did not know how to use sftp and asked for ftp but I use only sftp on my server because it is secure. And now after using plugin for some while and just paid 1 year subscription I will have to find another SEO plugin because this one is not usable when slowing my site down. Now I have to waist my time because authors of rank math do not care about their plugin or customers. I found out when googling that a lot of other people have same issue. I wish I knew this earlier , I would never had purchased this plugin so watch out before buying.