Memory leaks are not unusual - and can cause older devices that have less RAM to suffer reduced performance over time. Even more recent models with significantly more system resources (such as some models of i_ad Pro with 16GB RAM) can suffer from this phenomenon.
Sometimes force-closing some open Apps can improve performance. While force-closing Apps should not normallybe required, this can help.
Quit and reopen an app on iPad - Apple Support
A routine restart of the iPad will usually clear reduced performance. For older devices, this may be necessary more often than for devices with more system resources. Perhaps getting into the habit of performing a simple pre-emptive shut-down and restart (a forced-restart being unnecessary in this instance) will alleviate your problem - at least in the short term. A future software update may also reduce memory leaks.
You can perform shut-down from iPad settings:
Settings > General > Shut Down