It doesn't matter, look at kinesis. People are willing to use it over kafka (even though AWS has a hosted kafka solution), just because the percieved convenience and interconnectivity to other AWS services. If timescale starts to make serious money, then AWS can use this to mobilize their engineering and make it fast and take timescale out. Amazon's goal is literally to be the only kid on the block for everything for…
Performance is often transient and it isn't the only criteria used when choosing a vendor. Some enterprise customers prefer a single vendor, i.e., "one throat to choke". Others have very specific constraints: greenfield decision making is a luxury.
My impression is that Amazon responds to customer requests to solve specific problems and they make a decent effort to continuously improve the performance and other aspects of their services. At one point people said "no one gets fired for choosing IBM" and this switched to Microsoft and, rightly or wrongly, Amazon AWS now wears this crown.