The R38 lags for some people for a couple of reasons.
1: It's use of PNG files doesn't technically make it lag, it is the Alpha Channels used in said PNG files that offer nice shading affects that make it lag.
2: Nearly every light, button, handle, etc. can be manipulated in some way. There are no static buttons. That means a lot of textures need to be loaded by the CPU and rendered, some on top of each other to provide the desired affect.
3: The R38 uses panel2.cfg instead of panel.cfg coding. This allows for the use of a higher resolution cab texture that provides greater cab detail. The higher resolution textures may slow down graphics cards with a small amount of memory.
BTW the amount of cores your computer has doesn't really matter; OpenBVE doesn't really take advantage of more than one CPU core. Expect the R38 to lag a lot less once OpenBVE gets true multithreading support, as most of the lag in CPU-bound.
Anyway, back on topic.