When DIMM (DRAM) is in a high temperature environment, will the speed of DIMM (DRAM) automatically decrease due to the high temperature?

DIMMs (DRAM) do not throttle themselves automatically.

Throttling in DRAM is not triggered by the DRAM module itself but is managed by the controller in the motherboard system.

When system temperature becomes too high, the motherboard or CPU may adjust memory controller parameters or lower the system clock frequency to reduce heat generation and protect the hardware. These adjustments affect DRAM performance, but they are implemented by the system as a whole, not by the DRAM modules themselves.

If you are concerned about DIMM instability due to heat, consider choosing models with heat spreaders.