Form https://github.com/ValveSoftware/Proton/issues/1178 and https://github.com/doitsujin/dxvk/issues/622#issuecomment-419256413 .
It seems one of UE4 threads do constant directory scan in search of nvapi64.dll/nvapi64.dll.so

stat(".../SteamLibrary/steamapps/compatdata/414340/pfx/dosdevices/c:/windows/system32/nvapi64.dll", 0x171be330) = -1 ENOENT (No such file or directory)
stat(".../SteamLibrary/steamapps/compatdata/414340/pfx/dosdevices/c:/windows", {st_mode=S_IFDIR|0775, st_size=4096, ...}) = 0
stat(".../SteamLibrary/steamapps/compatdata/414340/pfx/dosdevices/c:/windows/system32", {st_mode=S_IFDIR|0775, st_size=49152, ...}) = 0
stat(".../SteamLibrary/steamapps/compatdata/414340/pfx/dosdevices/c:/windows/system32/nvapi64.dll", 0x171be3c0) = -1 ENOENT (No such file or directory)
openat(AT_FDCWD, ".../SteamLibrary/steamapps/compatdata/414340/pfx/dosdevices/c:/windows/system32", O_RDONLY|O_DIRECTORY) = 4529
ioctl(4529, VFAT_IOCTL_READDIR_BOTH, 0x121000) = -1 ENOTTY (Inappropriate ioctl for device)
close(4529)                             = 0
openat(AT_FDCWD, ".../SteamLibrary/steamapps/compatdata/414340/pfx/dosdevices/c:/windows/system32", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = 4529
fstat(4529, {st_mode=S_IFDIR|0775, st_size=49152, ...}) = 0
getdents(4529, /* 799 entries */, 32768) = 32752 << directory scan in inner loop!
getdents(4529, /* 86 entries */, 32768) = 3632
getdents(4529, /* 0 entries */, 32768)  = 0
close(4529)                             = 0

What affects performance.