A lot (all?) recent USB sticks have terrible thermal design, and will throttle seemingly arbitrarily to very low speeds under sustained load. Like 2.5MB/s type speeds. They seem like they were made to to theoretically exist for the market niche, but no one expected them to actually be used by anyone who paid any attention at all.
Same for ones bought in big box stores as Amazon or the like. Name brand or random brand.
A lot of less expensive 2.5+ Gig Ethernet dongles do the same.
Good performance for 5-10 seconds, then abysmal.
I switched to SD cards, and at least the good brands of those had decent and predictable performance (50-75MB/s sustained for the same price point). They were also a lot cheaper in general for the capacity.
