Difficult to say. Have you tried deleting your LW preferences to see if that helps: Deleting LW Preferences folder in OSX

Else I could point you to a different version of LW. Officially LW are not letting people downgrade 'if' their computers are capable of the most recent version.

There's other options such as turning performance options off. LW preferences -> Advanced -> SuperReallyAdvanced -> Performance.

Other than this, getting rid of incomplete files if you have too many. They can make some computers struggle. To do that with LW 5 you'd need to cancel each one (or hold shift key down from first to last & press cancel). Then check the incomplete folder to be sure none still exist inside.