Your Mac Pro 5,1 can run Mojave without OpenCore. OpenCore is mostly a boot loader, so its impacts are almost entirely gone by the time macOS is up and running,
By far the easiest way to cause poor performance, instability, overheating and crashing is to install ANY third-party speeder-uppers, Cleaners, Optimizers, or Virus scanners. or a VPN that you installed yourself.
The idea that a third party, with no special knowledge of the inner workings of MacOS, can somehow find a simple way to protect your computer — that is not already being done by MacOS itself — suggests that the MacOS developers are somehow "holding out on you". That is absurd.
You should remove any and all (other than Apple built-in) virus scanners, speeder uppers, optimizers, cleaners, App deleters or VPN packages you installed yourself, or anything of that ilk.
Third-party file Sync-ers such as DropBox, BackBlaze, OneDrive, or GoogleDrive can ruin performance, because they were incompletely PORTED from other environments. They also scan your local files non-stop, and your internet-stored files non-stop, looking for changed items. MacOS native file Sync-ers like iCloud drive use MacOS data structures like the File System Event Store to note what folders have changed instantly.
