Sounds like a few things could be going on either with your computer or with your internet here are a few things you can try to fix it. 1st is if you are having issues with textures/meshes you may want to check you graphics settings both in game and through the graphics card itself (either the settings you have are too high for the card to handle or are too low) also make sure you have a compatible graphics card, if you do then then either update the card or download and install any drivers needed for the game (drivers can be found on the manufacture's website), the 2nd thing to check is your internet speed At Bare Minimum the download speed needs to be at least 3mbps or better (you can go lower then this but you will have lag issues) also if the Download speed is below 1mbps then that may be the source of the issue and upload speed should be at least about 1~2 mbps or better (preferably better) also check to make sure you have a low ping too (preferably below 30ms) as a high ping can also cause lag and other problems (contact your ISP for more help with internet issues or upgrade your plan), 3rd thing you could try is to switch from a Static IP to a Dynamic IP (if possible) to see if that fixes the issue as it is very rare for anyone outside of businesses or webmasters to have a static one, 4th is to check to make sure your computer meets the minimum requirements to run Warframe (I know this is pretty basic but cover your bases as I always say) if it does then restart your computer in "Safemode With Networking" and launch the game in there (you may need to run/activate the Graphics Card first) and see if you are still having the same issue if the game works and nothing happens when you try to ALT+TAB then it is likely something else on your computer that is not playing nice with the game (ie anti-virus or firewall) and not warframe directly being at fault, 4th on that note make sure you have excluded any folder that warframe uses from your antivirus and make sure the game has the proper permissions to access the internet and make changes to the computer also you may have more then one firewall as in most cases both your computer And your modem/router has a firewall (if you have an external router check ports in both modem and router) so make sure the proper ports are opened in all of them, 5th if you did the port forwarding manually if so did you disable UPnP first as that can cause the Strict_NAT ERROR if not try disabling UPnP and redo the port forwarding manually, lastly you can do a full uninstall/reinstall of the game and make sure you are installing it to the C/: drive as doing custom installs or installing programs on other drives can cause programs to be unstable. If issues continue after trying all of the above then I have a few questions to ask you, what OS are you running the game on Mac/Win/Linux/Other? Did you install through Steam or from the Warframe website? Does this only happen when you ALT+TAB when in a mission, loading into one, or just anytime after you ALT+TAB? Can you install Warfarme on a different computer and if so do you have the issue on that one also? Are you using port forwarding and if so did you forward the right ports (https://portforward.com/help/warframe/ for correct ports by platform)? did you do the port forwarding manually or did you let the router do it for you? Does this still happen even without the Port Forwarding? Lastly DE Glen posted guide for dealing with the Strict_NAT ERROR https://forums.warframe.com/topic/13981-strict-nat/ keep in mind that this guide is old but it should still be useful.
Anyway I hope this helps or was at least useful to you.