My computer is completely crap so I'm familiar with the problem. Fully endorse all the tips already mentioned, here's a few more
- with careful positioning and playtesting you can often significantly reduce the number of units you need. Also you can use fewer units with higher skill.
- waypoints slow things down, so only have ones you really need
- triggers also. A trigger has to run all the time so use scripts whenever you can - they only run when you tell them to. Use logic to reduce the number of triggers you need
- aircraft are real a real problem
- vehicles are next: have as few types as possible ie if there are no US helicopters don't have 2 shilkas and 2 bmps defending the base, just have 4 bmps.
- lag is worst when there are lots of units close to the player. It's not so bad if they are far away
- it may help if you group static objects (tents, sandbags), not sure
- be pedantic about turning off background programmes (task scheduler, antivirus etc) ... but make sure you remember to turn them back on again later
- reboot before playing
My computer is below the minimum spec for Resistance and I managed to finish the campaign. Mind you, more than once I had to turn up the view distance, have a look, then turn it down again