after updating to a new IOS the system reorganizes itself (for example now completely all the existing photo library, music lbrary, etc) PLUS synchronizes all of it with your personal cloud, if activated also with ALL other apple products like iPhone, macbook, mac Pro, ...., often via WLAN... and at the end,all this will be backuped automatically for each product.
the User does NOT realze this bevause it happens "in the background", using only ressources taht are actually NOT needed by the user during this period. this period can last a long time, depending on the number of data, number of communicating (synchronized) products, ....
all this needs a lot of energy. Because of this your processors and your batteries will produce much more heat than in the period before - just until all these processes are done...
BTW this happens only when connectes with your charger.
so, I think this is the reason for your iPad getting warmer.
IMHO this is also the simple reason for the millions of people getting in a panic-status because "Mimimi... after updating my iPhone has less battery time".
just wait some days until all the background processing is done...
if you did a "clean upgrading" first disable all the energy-consuming options that are "on" by default...