First thing to do is disconnect the throttle cable from the engine and ensure that the engine throttle linkage is free.
Then operate the throttle with it disconnected to see if it is still stiff, if yes, then carefully inspect the cable run for cable damage, kinks, and sharp bends.
It could be the side remote causing the issue, but that is very unusual, to test it, disconnect the throttle cable at the remote and then operate the throttle to check for stiffness. Some side remotes are PITA to pull apart, and you need six pairs of hands for reassembly.
It is really a matter of elimination, however, if you identify the cable as stiff, then don't mess about, just replace it.