[SOLVED] Windows guests are extremely slow

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ProfessionalN00b

New Member
May 21, 2025
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Estonia
Hello

I came from Hyper-V last month where my VMs performed very well. The host server is the same, so I expected the same performance from a different hypervisor as well.

Is there some sort of best practice for Windows guests? That is current. I tried to follow whatever suggestions I could find on YouTube, but clearly none of it has helped.

This is a config of a domain controller VM:
Code: 
agent: 1
balloon: 2048
bios: ovmf
boot: order=scsi0
cores: 6
cpu: host,flags=+hv-evmcs;+aes
efidisk0: Storage:vm-100-disk-0,efitype=4m,pre-enrolled-keys=1,size=4M
machine: pc-q35-9.2+pve1,viommu=virtio
memory: 4096
meta: creation-qemu=9.2.0,ctime=1747927001
name: myVMname
net0: virtio=00:00:11:0D:00:17,bridge=vmbr0,tag=2
numa: 0
onboot: 1
ostype: win10
scsi0: Storage:vm-100-disk-2,backup=0,iothread=1,size=60G
scsihw: virtio-scsi-single
smbios1: uuid=0007f5b5-000c-0003-8000-bc739459000b
sockets: 1
startup: order=1
tags: dc;win
tpmstate0: Storage:vm-100-disk-1,size=4M,version=v2.0
vcpus: 1
vmgenid: 28498000-2000-4500-000d-673ac4000063

Let me know if I need to include something else.
---
Edit: I thought maybe adding more cores would help so I increased core count from 4 to 6 yesterday and added the cpu flags, it had no effect.
 
Last edited:
MarkusKo said:
i understand that you don't want to disable VBS but if you struggle with performance then that's most likely the culprit.

https://forum.proxmox.com/threads/t...-of-windows-when-the-cpu-type-is-host.163114/
Click to expand...
I changed the processor type from Host to x86-64-v2-AES. Reduced core count to 4 again and the performance is now at expected levels, but now VBS is enabled, but not running, which is not ok. Not only doesn't Proxmox support Kernel DMA, now I can't use VBS either.

I'm complaining because the performance degradation wasn't nearly as bad on Hyper-V with VBS. I understand that the real problem here is the older CPU. I've heard that performance drop can be up to 30% with mitigations, but I'm experiencing way above 50 with VBS enabled (which requires the Host processor type). It was only slightly better than a slideshow.
Can't go back to Hyper-V either because 2019 was the last free version.

I guess case closed :(
 
Wrote a small script to dected proper cpu type for VM configuration:


Bash: 
#!/usr/bin/env bash
# cputest.sh — Detect x86-64 micro-arch levels (v2/v3/v4) and show a tidy summary.
# Options:
#   --no-color   disable ANSI colors
#   --full       also print the full "common flags across all cores" list

set -euo pipefail

# ------------------------ Options & color ------------------------
USE_COLOR=1
SHOW_FULL=0
for arg in "$@"; do
  case "$arg" in
    --no-color) USE_COLOR=0 ;;
    --full)     SHOW_FULL=1 ;;
  esac
done

if [[ ! -t 1 ]]; then USE_COLOR=0; fi

if (( USE_COLOR )); then
  C_ORANGE=$'\e[38;5;208m'
  C_GRAY=$'\e[90m'
  C_GREEN=$'\e[32m'
  C_CYAN=$'\e[36m'
  C_RESET=$'\e[0m'
  C_BOLD=$'\e[1m'
else
  C_ORANGE=""; C_GRAY=""; C_GREEN=""; C_CYAN=""; C_RESET=""; C_BOLD=""
fi

# ------------------------ Header ------------------------
print_header() {
cat <<'EOF'
               P R O X M O X   V M   C P U   C H E C K

EOF
}

print_header
echo

# ------------------------ Collect flags ------------------------
mapfile -t all_flags < <(grep -E '^flags\s*:' /proc/cpuinfo | cut -d: -f2- | sed 's/^[[:space:]]*//')
if ((${#all_flags[@]}==0)); then
  echo "Could not read CPU flags from /proc/cpuinfo." >&2
  exit 1
fi

# Build intersection of flags across all cores
read -ra base <<< "${all_flags[0]}"
declare -A present
for f in "${base[@]}"; do present["$f"]=1; done
for ((i=1;i<${#all_flags[@]};i++)); do
  declare -A cur=()
  read -ra tmp <<< "${all_flags[$i]}"
  for f in "${tmp[@]}"; do cur["$f"]=1; done
  for f in "${!present[@]}"; do
    [[ -n "${cur[$f]:-}" ]] || unset 'present[$f]'
  done
done

has() { [[ -n "${present[$1]:-}" ]]; }
require_all() { for f in "$@"; do has "$f" || return 1; done; }

# ------------------------ Levels (psABI/GCC aligned) ------------------------
v2_req=(cx16 lahf_lm popcnt sse4_1 sse4_2 ssse3)
v3_req=(avx avx2 bmi1 bmi2 f16c fma movbe xsave)
has_lzcnt() { has lzcnt || has abm; }
v4_req=(avx512f avx512dq avx512cd avx512bw avx512vl)

level=1
if require_all "${v2_req[@]}"; then
  level=2
  if require_all "${v3_req[@]}" && has_lzcnt; then
    level=3
    if require_all "${v4_req[@]}"; then
      level=4
    fi
  fi
fi

cpu_model=$(grep -m1 '^model name' /proc/cpuinfo | cut -d: -f2- | sed 's/^[[:space:]]*//')

# ------------------------ Pretty printing helpers ------------------------
comma_join() { local IFS=", "; echo "$*"; }
print_cols() { # prints words in N columns (default 4)
  local COLS=${1:-4}; shift
  local i=0
  for w in "$@"; do
    printf "  %-14s" "$w"
    (( ++i % COLS == 0 )) && printf "\n"
  done
  (( i % COLS )) && printf "\n"
}

# ------------------------ Output ------------------------
echo "${C_BOLD}CPU:${C_RESET} $cpu_model"
case $level in
  4) echo "${C_BOLD}Detected level:${C_RESET} ${C_ORANGE}x86-64-v4${C_RESET}" ;;
  3) echo "${C_BOLD}Detected level:${C_RESET} ${C_GREEN}x86-64-v3${C_RESET}" ;;
  2) echo "${C_BOLD}Detected level:${C_RESET} ${C_CYAN}x86-64-v2${C_RESET}" ;;
  1) echo "${C_BOLD}Detected level:${C_RESET} x86-64-v1 baseline" ;;
esac
echo

echo "${C_BOLD}Key capabilities:${C_RESET}"
# Show a concise, readable summary
summary_flags=(avx avx2 fma f16c bmi1 bmi2 movbe lzcnt aes pclmulqdq sha_ni adx invpcid fsgsbase rdseed rtm hle)
have=(); for f in "${summary_flags[@]}"; do [[ -n "${present[$f]:-}" ]] && have+=("$f"); done
print_cols 6 "${have[@]}"
echo

echo "${C_BOLD}Level criteria check:${C_RESET}"
printf "  v2 requires: "; print_cols 6 "${v2_req[@]}"
printf "  v3 adds:     "; print_cols 6 "${v3_req[@]}" lzcnt
printf "  v4 adds:     "; print_cols 6 "${v4_req[@]}"
echo

if (( SHOW_FULL )); then
  echo "${C_BOLD}Common flags across all cores:${C_RESET}"
  mapfile -t all_present_sorted < <(printf '%s\n' "${!present[@]}" | sort)
  # print in 6 columns for compactness
  print_cols 6 "${all_present_sorted[@]}"
fi
exit 0
 
ProfessionalN00b said:
I changed the processor type from Host to x86-64-v2-AES. Reduced core count to 4 again and the performance is now at expected levels, but now VBS is enabled, but not running, which is not ok. Not only doesn't Proxmox support Kernel DMA, now I can't use VBS either.

I'm complaining because the performance degradation wasn't nearly as bad on Hyper-V with VBS. I understand that the real problem here is the older CPU. I've heard that performance drop can be up to 30% with mitigations, but I'm experiencing way above 50 with VBS enabled (which requires the Host processor type). It was only slightly better than a slideshow.
Can't go back to Hyper-V either because 2019 was the last free version.

I guess case closed :(
Click to expand...
Have you tried to add +vmx flag to the cpu line in your VM config to enable VBS again?
 
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