It will only cause a problem with slow shutterspeeds if you do not keep your finger pressing the shutter release button until the shutter has done and completed its job.
So do exactly that!
You should do that at all speeds, but it becomes really important at speeds of 1/15 and slower.
The T-lever is fine for long exposures, with the shutter set to "B".
It will keep the rear shutter open, but to close it, you need to set it to O again (just like a real T-mode, hence the "T".)
A cable release would be better though. The T-lever will not work with a cable releases, so you need one that has a lock of its own.
P.S.
Get yourself
a User Manual for your camera !