Turning off STP speeds up wifi connecting even if just one Mikrotik device.
Yes it does. If the wifi interface had no other connections. The WLAN interface state transition starts a spanning tree convergence in the bridge. Just one Mikrotik device can be a network topology on its own. And normally is connected to something else, what makes STP have to select a root-switch and rebuild MAC tables and enable forwarding on interfaces.
But why did you set STP? The default is RSTP, a much faster protocol, that will not introduce that convergence. If you deviate from the default, you should have a reason.
See the wiki: "STP is considered to be outdated and slow, it has been almost entirely replaced in all network topologies by RSTP, which is backwards compatible with STP."
On just one Mikrotik device , STP protocol should be "none" indeed.