To narrow down the search, I took a look at how the flashlight was programmed in client.qc. It turns out that the flashlight uses different effects in the flash_on function between versions U and T.

In version T:

myflash.effects = EF_DIMLIGHT;

In version U:

//myflash.effects = EF_DIMLIGHT;
myflash.traileffectnum = particleeffectnum("flashlight"); //tribal glow

Presumably the fancy effects are not compatible with high frame rates in the engine, and changing back to EF_DIMLIGHT will fix the issue.