Assuming that you recently triggered Workflow in your system hence the number of users shot up three times. At peak hours the system is terribly slow, nobody can work. 

What short-term solutions (SAP fine tuning etc) are there as you plan to upgrade your hardware? 
 

This depends on what are doing with your workflows. 
If it's little workflows that start often put the events in queues.

Is it a custom workflows or standard workflows?
If custom, try to replace methods by attributs (no RFCs) and it's better to use virtual attributs (only calculated when accessed).

If you use heavy BDC or run heavy reports in your workflows you could try to give more ressources to the CPIC or background user.

If your workflows have a lot of background steps before sending workitems to users, and if possible, make them start during the night.

And of course, if all this doesn't solve the problem you will have to your upgrade hardware. But before that, you should  really dig in to see if there isn't a design/configuration problem somewhere:

SWI5: workflows workload analysis 
Put traces on workflows and events for example.

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