It is funny with all these new operating systems supposed to be faster on the old devices; yet less and less of these old devices can actually run them.
These reviews claiming otherwise are pure marketing.
OS improvements usually come with a different cost: new bloat. Hence at some point your old hardware will not work good enough with the new OS. Or whoever made the hardware has no desire to support the new OS once they have sold you the device.
It is more obvious with Apple devices, although that can be attributed to a desire to force millions of devices becoming obsolete, so that new can be sold.
Another truth is that some tablets with Tegra chipsets newer ran any Android well enough as CPU performance was bad, so no OS can fix that. no matter how many ROMs you try, overclocking, etc.