Well it's hard to say anything too definitive without knowing more about your setup but usually to display the output of an egpu on the internal screen of a laptop you have to copy the framebuffer back off of the egpu over whatever interface it's connected via (ExpressCard in your case, which isn't really that fast, basically a single PCIe 2.0 lane at best) and onto the gpu attached to the internal display so it can be displayed there. There's overhead in doing that, in multiple stages of the process, and also when using a slow interface like ExpressCard it could cut significantly into the PCIe bandwidth that you would otherwise be using for everything else the gpu has to do, lowering performance in your game.
It's hard to imagine that the limit for how fast you could copy the framebuffer would be as low as 60-ish fps for normal resolutions on any somewhat recent (past decade or so) hardware, so maybe it's worth going through the display settings for the gpu to which the laptop screen is connected and making sure there's no vsync forced in there or something, but ExpressCard bandwidth is very low so it's possible you're just really running into that limit.
The best test you could do would be to just try a monitor plugged directly into the egpu if at all possible and to check whether the issue is still occuring or whether it's remedied by doing that. You could also try a very small resolution like 640x480 with your current setup and see if that has any effect, as it would require much less bandwidth to copy than a larger resolution.