USB is purely pooled by the host, no Bus Mastering, no DMA. Bunch of frequent interrupts (125us uSOF) that need to be handled by the CPU. USB 2.0 is so heavy its mere presence in a computer (idle pooling something plugged in) visibly slows down any <1GHz computer (halving IDE transfers for example https://www.vogons.org/viewtopic.php?t=89651). Users didnt notice because 2.0 started showing up in 2002 together with >2GHz CPUs.