With CCleaner versions 5.10 through 5.14 on 3 PCs with Win8.1 x64, enabling "Run CCleaner when the computer starts" on the admin account results in the CCleaner taskbar icon running for 30 seconds. On a standard/limited account, enabling the same option results in the CCleaner taskbar icon running for 1 second.
All 3 PCs have solid-state drives, and they are fast modern PCs. I use Firefox as my browser, but disabling the cleaning of Firefox did not change the 30-second slow down. Disabling Windows Defender and uninstalling all anti-virus software did not avoid the slow down either. My company's firewall prevents CCleaner from initiating internet communications (in case CCleaner is waiting for internet feedback). All CCleaner monitoring options are disabled.
CCleaner v5.05 was last version I tested where Win8.1 x64 did not have the extra 29 seconds for admin account logon. On my Win7 x64 PC, all versions of CCleaner mentioned take 1 second on the admin account, just like the standard account.