While the network connection may be same, in comparing the relative performance of your iPad and iPhone, you are comparing Apples and Oranges. Such a test, in isolation, is inconclusive; comparison can be reasonably expected to show measurable differences. Further tests would be necessary to isolate a potential problem.
If instead you were to observe a performance deficit when comparing otherwise identical devices, then you might have reasonable grounds to immediately suspect a potential problem with the slower device.
While a network speed-test may indicate the network connection to be similar in performance, the internal system architecture of the iPad and iPhone differ.
Perhaps perform a forced-restart of your iPad and iPhone - as this will often clear an isolated software issue:
Restart your iPad - Apple Support
Force restart iPhone - Apple Support
If you still have concerns, you might be best advised to have your devices professionally assessed and tested. You can contact the Support Team using the Get Support link at top-right of this page. Alternatively, you can initiate and manage your support cases using the excellent Apple Support App. If not already installed on your iPad, the App can be downloaded from the App Store:
https://apps.apple.com/gb/app/apple-support/id1130498044
You might also visit the Genius Bar at your local Apple Store or Apple Authorised Service Provider (AASP) - where the technicians will be able to test and assess your iPad and iPhone:
Genius Bar Reservation and Apple Support Options - Apple
