I think the only thing you can do is buying an i7. The game itself is CPU intensive. RPH and LSPDFR also need more horsepower.
I have the same problem every time. I'm using lot of plugins like every Albo's plugins and also like you, ALPR+ (this is the worse plugin for me when driving in the city). Those plugins are doing a lot of background work so...
I also have an i5 (3570k @ 4500mhz) and a GTX 1070. There is nothing you can really do :)