Taken straight from the wiki:
Multiplayer Decal Limit
The maximum number of decals that can exist at once. Decals are used
for visual effects like bullet impacts. Higher numbers will result in
more persistent decals, at the expense of performance.
There is a command that represents the Multiplayer Decal Limit, noted by the command
mp_decals x with x denoting the amount of decals you want visible in game.
Since decals are responsible for showing sprays, bullet holes, and other particle effects in game, reducing x will in turn lower the amount of particle effects. X can be a number from
0 - 200.00.
If you'd like to have some particle effects (to still show bullet holes, useful for learning recoil patterns and such), an advisable amount is
mp_decals 9, which is enough for seeing the Scattergun's spread without sacrificing any performance. In Chris' words (creator of Chris' configs)
mp_decals 1 // `9' is a good value to still see the spread pattern from a
// scattergun without any real performance loss.