If it were the OS, then essentially everyone would be having this issue. Clearly, that is not the case or mention of it would be everywhere.
You may have third party software installed that is stepping on keystrokes they are not supposed to use. Or, it could be a hardware fault. Or, weak batteries in the keyboard.
Start the Mac up in Safe Mode. It's the only way to eliminate all third party possibilities as the problem could be being caused items in the root Library's LaunchDaemon and LaunchAgents folders, which will load even under a brand new user account.
In Safe Mode, try copy/paste at random intervals for an hour or so. Or, however long and often you feel would prove, or disprove the test. Does the problem persist?
If no, then something in your normal account is interfering with the system. If yes, then there's a hardware issue of some sort. It may be as simple as nearly dead batteries in the keyboard. Also, try removing the keyboard from Bluetooth, then add it again.
Similar to depleted batteries in the keyboard, the batteries may be going bad (not holding a charge). Try the keyboard with the charging cable plugged in. Does the keyboard work as expected when it's always getting external power, but fails when unplugged?