There is some evidence that the use of explicit primitives in Rules slows down the parsing of your Rules. I can’t say for certain this is in fact a true thing, but it might be worth spending some time going through your Rules and avoiding the use of int, long, float, double, intValue, longValue, etc except where absolutely necessary (i.e. when passing the number to a method that requires the primitive.
Letting the Rules DSL treat them as Numbers (BigDecimal to be specific) seems to be easier for the parser to work with.
I’ve only anecdotal evidence that this is in fact a problem but it has drastically reduced the load times for at least a couple of users.
Also, the nearly 2 to 1 ratio between Items and Rules seems off to me. Do you have a lot of duplicated code? If so, reducing the number of Rules and number of lines of code by using fewer generically written rules will also reduce your load times.
Following Markus’s advice is certainly going to address your immediate problem, but I’m constantly amazed at how patient OH users on RPis are when it comes to how long it takes to restart OH and load the configs. The advice I provided above might address that a little.