If this is just in the simulator for ios 11 or 10.3, then it is a known bug. The XCode 9 simulator is working very slowly for everyone relating to mapKit. This bug can be seen here and here. It should, however, work fine if you use the app on your phone.
Apple staff working with Swift development have acknowledged the issue. The only workaround currently is using the ios 10 simulator or changing the rendering modules (suggested by Apple staff). Supposedly, changing the modules isnt perfect, but it might just help. Type this in terminal:
defaults write com.apple.CoreSimulator.IndigoFramebufferServices FramebufferRendererHint X
Replace the X with: 0 = auto, 1 = Metal, 2 = OpenCL, 3 = OpenGL
Again, this is an official bug in the simulator so changing the rendering still won't make the simulation perfect. There have been 4-5 people who have submitted bug reports on this and it has been acknowledged, so hopefully a fix comes soon.
Edit
I tried a map kit app on iOS 11.1 simulator. It seems to be slightly smoother but the error message still pops up... waiting for a fix
Edit
A comment pointed out that this is fixed in the iOS 11.3 beta.
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