Since upgrading to v12 Media Encoder has become pretty useless. I'm working on the same projects as before, nothing has changed in terms of plugins or hardware I'm using.... but the render time has gone up from several hours (60min piece) to 2days. I used to be able to export a whole version overnight. Not anymore since v12 unfortunately.
Sorry for the inconvenience. Please share us more info related to the system configuration. Are you on Windows or Mac? What is the GPU model number? Have you updated the GPU drivers?
Is this happening with just with projects created in older version & converted to the latest version or with new projects as well?