Before anything else, verify the game files through the launcher — corrupted or missing files after an update are the most common culprit. Open the launcher, go to settings, and hit 'Check Files'. Also make sure your graphics drivers are up to date; War Thunder is sensitive to outdated GPU drivers and will silently crash on startup.
Lag in matches usually comes down to your connection to the specific regional server. Check which server region is selected in the launcher settings — sometimes after an update it resets to a distant region. If ping spikes happen mid-battle but your general internet is fine, try switching from Wi-Fi to a wired connection. Wireless interference is responsible for more packet loss than people expect.
If you're getting authentication errors on the Gaijin.net account page, clear your browser cache and cookies first. For the launcher itself, try logging out completely, deleting the login token file from the game's config folder, and signing in again. Two-factor authentication issues are usually caused by a time sync mismatch on your device — make sure your system clock is set to automatic.
This is almost always a RAM or VRAM issue. War Thunder's graphic settings can silently push your hardware over the limit. Lower texture quality one step and disable SSAO — these two settings have the biggest memory impact. Also check that no background apps are eating RAM while you play.
Delete the 'updates' folder inside the game directory and restart the launcher. It'll re-download only what's needed. If the launcher itself won't update, reinstall it without touching the main game folder — your settings and progress stay intact.