Slow wmts requests

Response time depends on many parameters - how large requests you are making, how complicated are, as well as the overall load of the system. In vast majority of cases, requests are handled in 0.5-1 seconds.
I checked your account and noticed that most of the responses are done in under 0.5 seconds:

I find this pretty fast, if you take into account that for each request we have to find relevant data among the total volume of 10 PB, decode the relevant information, create a composite based on your configuration, stich neighbouring tiles and reproject everything to your CRS of choice.

I notice that you are asking for data in PNG output format, which makes them quite large. Latency and download speed on your side may also play their role in perceived response time. I recommend you use JPG, making files smaller and therefore faster to download.

Output Formats

Requests provide output as image or in vector, and the Sentinel-2 WMS/WMTS/WCS services can generate these outputs in different formats.