This is untrue. Java is very fast once the JIT warms up. The biggest cost to java is the startup time, but for a long running application it's not really an issue.
In fact, Java can be faster than C for a long running application, because you avoid problems like memory fragmentation. Garbage collection can compact used objects into smaller sets of memory, improving cache utilization.
The parent comment said that they need to write it in C in order for it to be fast enough for people to use it, which is false.
As for timing attacks, it's very very hard to write sidechannel resistant code in any language, I don't believe Java is particularly harder. C/C++ is not as deterministic as you would think.
In fact, Java can be faster than C for a long running application, because you avoid problems like memory fragmentation. Garbage collection can compact used objects into smaller sets of memory, improving cache utilization.