Speculative execution isn't an inherent security risk, but it typically entails a cache fill, which can, if not cleaned up or not isolated, can be used for a timing attack. That's the real issue here: that execution results in mutating a shared datastore, and facts about the likely contents of that datastore can be inferred from performing additional operations and self-measuring aspects of one's own performance.

In the larger sense, mixing various privilege levels on shared hardware is likely a bad idea, despite being fundamental to general-purpose computing. This is because it's fairly difficult to cloak intrinsic "physical" attributes of execution, like execution timing, cache timing, from other processes, and essentially impossible (and/or unreasonable) to cloak it from one's own process. It is both possible for a process to generate lots of side-effects (e.g. IO, cache fill), and for it or another process to try to figure out ways the shared state changes by observing its own execution.