Peer-reviewing process of an article
"Under review" for nine months is normal in some disciplines


  Do you think I can stop the reviewing process and send it to another
  journal?


No. This will most likely delay the process even further.


  What if I sent it to another journal at the same time? 


No. This is (most likely) prohibited.


  Or, would the reviewing process speeds up if, e.g. I pay for the open access?


No. Open access is entirely independent of the reviewing process. The former is handled by the publisher, the latter is handled by your peers.