So, the UKs youngest Billionaire (i gather) who owns Bitmex, can't afford to upgrade his computer.
If deribit can handle more orders per second - it shows it can be done. It also proves Bitmex have a keen interest in not fixing it. (sure we all knew that ...)
So, Bitmex use a trading desk against their customers... and they give us a criminally underpowered system - guaranteed to break as soon as any volume comes in.... designed to break/overload if you believe the above quote.
It all comes down to the serverload. Deribit can handle more transactions per second because their servers are less stressed than Bitmex's servers. If you're a relatively smaller pocket trader, Deribit is a very decent alternative, but if you are a whale you will need Bitmex's depth to have your positions fill.
Sure, one could say that they should upgrade their servers and trading engine to allow more transaction throughput, which is a fair point, but this is something that can take a good year to tweak, because every change, regardless of how subtle it is, can change the user experience significantly.
As for their trading desk, it's quite odd that they openly admit to actually have their own market makers trading, but Deribit may have one operating too, which they don't tell anyone about. I by default assume that regardless of the exchange I use (spot or derivatives), the exchange is trading against its users. Why wouldn't they? They can front run anyone's orders, see how much fiat is entering or leaving, etc.