Is it difficult in releasing the lock, or is the pivot just too tight? Or both?
Either way, I think the first thing I'd do is thoroughly flush out the mechanism (pivot and lock) with WD-40, and vigorously exercise the joint & lock while doing so. If this is an old knife, and especially if it's carbon steel(?), there may be some very old, sticky oil and/or rust in the pivot/joints. That could contribute to the extreme stiffness of opening/closing. WD-40 will do a pretty good job in loosening it up, and flushing out the gummy, dirty 'gunk'.