Indeed, my recommendation is to avoid the use of primitives as much as is possible. But it’s not possible to avoid them completely because Java library classes like Math and even Joda DateTime’s plus/minus functions all require the use of primitives. Constants like the numbers used in the calculations above should be instantiated as BigDecimal for you so it’s the calls to xValue (e.g. floatValue) or defining a var/val explicitly as a primitive that will force the Rules DSL to use primitives.
However, I’ve not seen reports of Rules taking minutes to run. All previous reports had to do with how long it takes to load and parse the Rules (I’ve seen reports of elimination of primitives shaved 15 minutes off of rules load times on an RPi). Once parsed though, the Rules would run just fine. This extended load time can get exacerbated if you are in a tight edit and save loop as your changes are likely to be faster than the machine can load and parse the Rules, hence Markus’s recommendation.
OP is running 2.5 M3 (unless I missed something) so whatever changed happened two+ months ago. I’m surprised there hasn’t been more users reporting the same behavior. And I haven’t watched the PRs that closely, has there been any updates to the Xtend/Xtext libraries in that time frame? That’s about the only place where this problem would reside, and it means we will need to wait for the upstream to fix whatever they broke. 
This behavior appears to be something new and needs more exploration I think.
I’m actually not seeing this behavior (on the latest snapshot build).
The following Rule:
rule "Test primitives"
when
System started
then
val long start = now.millis
val double num = 100
val calculation = Math.log(num.doubleValue)
val long stop = now.millis
logInfo("Test", "The Rule took " + (stop - start) + " milliseconds to run")
end
Outputs
2019-10-22 15:51:45.094 [INFO ] [.eclipse.smarthome.model.script.Test] - The Rule took 2 milliseconds to run
with no spike in CPU.
So the behavior is not universal which is going to make diagnosing it a bear.
As an aside, there is a third party Weather Calculations Binding that may have this calculation built in.
I could also see calculations like this made as library functions in the Helper Libraries.