lag in tf2? impossible!
this is unheard of!

I don't know if you regularly hit 720 frames per second, but hitting the fps cap can cause stuttering. I would suggest setting fps_max to 0 and see if that helps at all.

Are you using flashing crosshairs or damage indicators?

Nope

Microstutters are caused by major fluctuation in frame times.

I'm going to cover a bit more than what would apply to you just incase anyone else finds this thread.

Thermal Throttling

More or less hardware overheats loses some efficiency, probably underclocks itself to prevent damage and causes a massive loss in framerate causing higher frame times. Identifying what is overheating can be a bit of a pain especially with laptops there's a ton of useful tools for finding what is overheating. In the past I've used Speedfan you may want to find something more regularly updated that that though.

Possible Fixes

• Clean your PC with compressed air getting as much dirt, dust and hair out of it as possible
  
• Make sure your PC has access to air. I.e. don't put it in tight/confined places and don't let clutter build up around it.
  
• Add and or Replace Case Fans Installing Case Fans
  
• Laptop users, make sure your laptop is on a flat surface and preferably use a cooling pad.
  
• While rare you may need to replace the CPU heatsink fan
  
• ADVANCED USERS ONLY replace your cooling system for your CPU/GPU with a better one, or clean and reapply thermal paste.
  
• OVERCLOCKERS if for some reason you didn't disable the thermal throttling controls and you're getting throttled your cooling system is inadequate for the overclock you're trying to achieve and you need to backoff or improve your cooling.
  
• ADVANCED LAPTOP USERS you may want to look into ThrottleStop Has a lot of neat features and shows you CPU and GPU temps with some stress test tools plus a lot of other controls that you may not have access to in BIOS. Make sure you readup on what's what before you go to crazy.

Power Throttling

Either A, not enough power is getting to the hardware or B. The Hardware has reached its max TDP (it's a pain to find this, but this doesn't normally happen pretty much only happening when users overclock in which case you just have to accept it and back off a little)

Possible Fixes

• Use High Performance power options / disable low power options where applicable. Win10 High Performance Adjust power settings for Max Performance in Video Card drivers as well. Note laptop users should only do this while plugged into power.
  
• ADVANCED USERS ONLY Disabling C states in BIOS (if possible) Note technically not a true power throttle does cause some latency when enabled though

There's a few more things to add, but most of them are caused by Thermal Throttling in which case see above, weak PSU in which case you'd have a whole lot more than just microstutters going on, or failing hardware which again you'd have a more issues than just microstutters. All of which are usually a pain to determine.

Poor Optimization

This applies to a whole lot of things and not just what game your playing. Closing Background programs and adjusting various performance settings both in your OS and in game can help a lot. You could be experiencing various driver issues Ex. BF4 at least used to have an issue with a lot of video cards if you didn't fully uninstall the old driver before installing the updated driver and it would cause a major memory loop. The game you're playing could be designed in an era that basically every CPU in existence had 2 or less Cores/Threads Ex. Source Engine Games. The game you're playing was designed by people who didn't give 2 shits about LOD models, etc.

Possible Fixes

• Lookup "Perfromance Optimization" for your OS. Any I could link here would be at least partially outdated in a few months.
  
• Closing out of all background programs when playing
  
• Fully uninstalling old drivers and/or installing Updated drivers Display Driver Uninstall tool; Snappy Driver Installer great tool for finding driver updates for most of your hardware. Make sure you use the SDI Lite version and download the indexes and only the indexes first, skip drivers that you've already update manually i.e. Video Card Drivers.
  
• Use FPS configs / Lower settings, or mods/edits that improve performance / disable whatever is causing the microstutter

Workarounds if you're still having issues after going over the above

• Setting a FPS cap just above your minimum frame rate. Frame time can't variate a whole lot if you're already near your minimum FPS. Can also prevent significant CPU/GPU usage and you might notice your minimum FPS is higher since your hardware doesn't get as hot or use as much power. This can also be done by using VSync where your FPS should stay locked at your refreshrate (only do this if your FPS rarely dips below your refresh rate and you don't care about Input lag) GSync/FreeSync (I can't fully explain this, but basically you want your minimum fps to be above the minimum refresh rate)
  
• Adjusting Pre Rendered Frames (Nvidia) or Flip_Queue_Size (AMD) you will need to do research on finding a way to adjust flip_queue_size as I'm finding a lot of material I know is out of date. Note Pre-Rendered Frames is the number of Frames generated by the CPU before it's outputted by the GPU (minimum setting of 1 as 1 frame has to exist to be outputted). Lower Pre-Rendered Frames = Lower Input Lag, but some games have lower minimum and average FPS and microstutters become more visible. Higher Pre-Rendered Frames may help boost your FPS up a bit and will help smooth out major FPS drops at the cost of higher Input Lag
  
• Getting your Minimum FPS so high it doesn't matter if you drop to it. Basically you have 700FPS and you suddenly drop to 300 fps and never go below that. (this kind of drop would cause only a ~1.8ms microstutter for reference dropping from 75fps to 60fps would cause a ~3.3ms microstutter ) This usually means disabling a lot of graphical options going to the lowest settings possible, and more than likely overclocking.

fps_max non0 can cause stutters for some PCs and not others with otherwise the exact same settings. So your mileage may vary on using that command.

Hope this helps. Can also give you a few tips and pointers to get you going in the right direction for the kind of play you want.

Do you want your game to be on Super Ultra settings, or do you want to try to be Invite level and don't care about graphics?

Are you knowledgeable about overclocking and having the Cooling (and motherboard) that allows for it? If you don't know the answer to any part of this the answer is No.

Have you gone through your OS and use settings for the best performance or best trade-off for you? If No, do this before anything else.

Guys I've found a solution. Thanks everyone for posting/upvoting etc.

It was my projectile sapper particles that were causing the lag. That's why I noticed the issue more on soldier though on scout when sent airborne (BY A PROJECTILE) the issue persisted. The game runs fine now; this has been bugging me for ages!

For reference:
"ExplosionEffect" "ExplosionCore_sapperdestroyed"
"ExplosionPlayerEffect" "ExplosionCore_sapperdestroyed"
"ExplosionWaterEffect" "ExplosionCore_sapperdestroyed"

If you still want to use a replaced explosion effect you can try this one.

https://www.dropbox.com/s/lpqxcjszoqzrjrk/explosions_muzzle_flash.vpk?dl=0

hope that one doesn't cause you microstutters.

Thanks very much! I've tested it and so far it seems to be working just fine.