How can I prevent apps from starting on iPad?

When you start an iPad after powering it off it does not start any apps; killing apps is one advantage of power cycling. How long is “too long”? Do you have any apps that claim to clean, protect against “viruses” or that claim to speed up? Or do you use VPN? All of these will paradoxically slow down your device.

Some understanding of how memory management works within iOS/iPadOS might help comprehension of how your iPad operates. To be clear, this is not intended to be an in-depth explanation, but will hopefully capture the essentials in basic terms...:

Your iPad/iPhone will always attempt to use most of its available RAM - unused RAM in this low power/high performance system architecture is essentially wasted resource. 

iOS Apps are generally in one of four states - the first three are the most relevant.

• The App is “Active” - it is running running in the foreground. When you switch tasks, the App will continue to run in active state for some minutes before its resources are released and is placed into an Inactive state.
• The App is “Inactive” but remains loaded in [fast] RAM. In this state, the App can be instantly restored to an Active state - but is not consuming CPU or other resources whilst in the inactive state.
• The App is “Inactive” and unloaded. In this state, the App has been completely offloaded (releasing RAM for use by other processes) but its running state has been saved to [slow] flash memory. Returning to an App in this state will cause the App’s saved state to be reloaded from flash memory into Active RAM - without the need to re-initialise the App.
• The App has been closed. All running data has been expelled - there is no “saved” state; relaunch will reload and re-initialise the App from scratch.

Memory management is generally a juggling act - and for the most part, you should allow the OS to manage its memory space. Re-initialising an App is the slowest and most power hungry method of launching an App. Inactive Apps do consume some system resources, but by design, the required system resources to maintain this state are insignificant.

Force-closing an App releases all resources. Whilst the mechanism exists to do so (and in rare circumstances manual intervention to terminate an App may be necessary), a force close reduces system performance.

If you choose to shut-down or simply restart your iPad, the iPad will save the current state of initialised (but not necessarily running) Apps. Upon restart, the machine state is essentially restored, re-loading previously active Apps to RAM. This takes a little time to efficiently reload.

By contrast, a forced-restart effectively disconnects power without providing opportunity to save current running state. In this condition the Operating System itself must be fully reinitialised; while useful to clear any faulting processes, this is not power efficient. All Apps must be similarly reinitialised, which while seeming to be faster, is power-inefficient.

Perhaps consider that your iPad is not attempting to emulate the performance of a desktop computer - but instead is trying to operate as power-efficiently as possible while providing high performance when needed.

Modern devices are not really meant to fully turned off when not in use, so few are really optimised for a quick full restart

one just suspend them when not in use, and put them to the charger when need be, most only fully turn off or restart them if they are updated or if something is wrong with them.

Excellent! User Tip material.