This is not true and it's a common misconception.
Miners do not perform complex calculations or solve complex mathematical problems as you would read on most crypto related websites.
Miners simply compute the hash of the block header twice: (sha256(sha256(block header))) and they keep doing this till they find a winning hash.
What makes mining hard and time/power consuming is the difficulty which is adjusted according to the network hashing power.
Miners or at least most of them do not use computers (CPUs) to mine anymore, they rather use GPUs or ASICs.