Sony SGPT21 - Manual (Page 141)

Your tablet device gets slow/is not performing as expected.
Your tablet device may slow down when you use it in a hot or cold environment. Use the tablet device in a
place with as suitable a temperature as possible.
Your tablet device may perform better if you turn off the tablet device, and then turn it on again when the tablet
device gets slow or is not performing as expected.
When your tablet device is not performing as expected, the data storage area of the tablet device may be full.
Uninstall any unnecessary applications.