No, there is no issue with ASA interfaces configured in sub-interfaces.
However, since it's a shared physical interface for 5 networks, I would check that the speed/duplex has been set correctly, and also if there is any errors on the interface.
Typically if the interface is overloaded because it's been shared by 5 different interfaces, slowness might be reported. So it's a good idea to check the interface counter.
I also assume that all the LAN traffic between the sub-interfaces, as well as towards the Inside and Internet from those 5 networks also traverse through the ASA, so if there are lots of those traffic, it might impact on the speed as well.