Slowdown in emulators is actually desirable. I know this might seem like a counterintuitive statement, but as an emulator becomes more accurate, it will need to be able to reproduce hardware specific slowdown by necessity. This led to some emulators being chosen over others because they did not accurately recreate hardware performance (such as ZSNES), thereby avoiding hardware accurate slowdown.
Its possible that in this case, the emulator is accurately recreating what hardware would be experiencing. Its also possible that your hardware running the emulator has a specific instruction incompatibility. One example of this is running Switch emulated games on an old Intel i5 with integrated graphics. The games should run fine, but the integrated graphics have a driver issue within the Vulkan drivers which causes severe slowdown. As such, only simple games like visual novels or 2D heavy games will run at full speed.