You can also try a wipe and factory reset from the recovery console if you don't want to flash a firmware.
With the S905 and S906 chipsets the good thing is that basically every box with the same amount of memory and same Wi Fi chip will be compatible for a firmware donation.
Over time things can get slow or after some "tuning" certain parts not longer respond as the should.
With most of these boxes already rooted and open to all sorts of attack people even ruin their Android system by the use of tuning tools designed for smartphones....
Considering their release strategy and time stamping procedure the firmware was created sometime Feb- March 2016.