Is this to avoid wear/tear on HDD because USB also suffers from wear/tear, MTBF, limited read/write cycles and potentially prone to NAND failure from irregular USB voltage spikes. If it's purely for speed, then I agree USB would likely win over HDD if HDD did have to spin-up first (ie not already in use by likes of timeshift).
I guess it really depends on individual circumstances/setup...
Anyone who has timeshift enabled then they are already using HDD so no difference and in fact you would be putting undue stress/wear/tear on USB stick for no reason imho. I've had my HDD in my solo2 for many years and timeshift and record a lot without an issue with HDD.