Does it reduces or makes the processing speed of cpu and gpu lower if i upgrade my ios to latest version in my ipad 8th gen?

You should always update your devices to the most recent release of iOS/iPadOS - as updates include bug fixes and essential security updates. Failure to update leaves your devices vulnerable to malware and other system security vulnerabilities.

Apple does not intentionally reduce performance of its products through updates - however, you need to recognise that new features can make greater demands upon device hardware. These additional demands can, where limitations of CPU and GPU are met, have impact on overall system performance. This phenomenon is not limited to your iPad and iPhone, but is common to all computing platforms.

For iPhone, with limited battery capacity, Apple has enhanced its system software with the intent of assuring system stability if power demands made of an ageing battery exceed its ability to provide sufficient power. This mechanism is intended to avoid issues with unexpected shut-down, instability or crash. This feature can be disabled, however, disabling protective software measures can have undesirable consequences.

You have heard is wrong and are you are very mistaken in your assumptions.

You have a fairly new iPad model.

If you have already an iPad on iPadOS 14.6, 14.8 maybe out very soon.

You can simply wait for iOS 14.8 or iPadOS 15 should be coming soon, as well, and you could download and install this new iPadOS version when Apple releases it sometime this September.

OR

Do nothing,

The choices are yours.

Good Luck!`