there is known bug which leads to performance degrade and even crashes

when you use non-default or obsolete allocators
to resolve it, do not use any other allocator than default

in the Arma 3 Launcher > Parameters > Advanced > Malloc
uncheck the field
(that's same as select Intel TBB 4 allocator)

and save

note:
possible ways to force allocators
1. Arma 3 launcher (mentioned above)
2. startup parameters in STEAMclient game profile
3. custom command-line passed to STEAM via 3rd party launcher

the issue is known, tracked and work in progress ...

same sticky in General discussions : http://gtm.steamproxy.vip/app/107410/discussions/0/364039785156643919/