It looks like the Brussels–prague train had a particularly bad patch in august, visible on zugfinder i think because it only runs every other day. It's not possible to look back that far for other trains. Was that one of the periods when there were a lot of problems caused by heat & speed restrictions?
Curiously, the times on zugfinder for the opposite direction don't seem (quite) so bad.
It would be interesting to know what is causing the delays (different issues on different days no doubt) and which ones are theoretically within ES's control.
(E.g. if one of the locomotives they use has failed you might argue that they should use a more reliable company, but if their train is blocked by a train ahead failing I don't see what they could do).
I do note the following from their FAQs, which isn't entirely reassuring:
https://www.europeansleeper.eu/faq
Are we reliable? Most of the time — though we’re still a young company, ironing out the wrinkles of international rail. Expect a bit of adventure, and know that we’re improving with every journey.