If you set to use 100% time and 100% cores, you are most likely overheating and your CPUs are throttling back to 50%, 25%, or even 10% of maximum until the cores cool off. Also, if you use the CPU for other things, that will reduce actual time spent on projects. Get a temp measuring program and monitor temps and/or monitor CPU clock. If this is happening, you may get better results by setting to 95%, or even lower, so it does not overheat. External cooling fans may also help.
This has been my experience on two different laptops.

First Rule: If "Run Always" has been selected in the client activity menu, BOINC will use all ''spare'' cycles of all the cores/threads allowed to run. If both 100% CPU time / 100% of Processors is shown in the Local Preferences screen then the client is tuned to the practical ''easy'' max.

IF those are the settings in effect, then it's a case of visiting the Result Status page to see the outcome of the jobs. Also compare the recorded Elapsed times [wallclock] in the client with the CPU time in task properties to see the system efficiency. A tool such as BOINCTasks is excellent to monitor to monitor this factor [and lots more]

Desktops wont back-off quickly when it gets hot, fans will start to howl, but laptops will slow the CPU itself if it does. I'd not meddle with that feature on those portable systems. Of course, dust-bunny removal every 2-6 months is a given, summer more frequent than winter and given the persisting droughts in for instance large regions in the US [the wider dust-bowl], that's where de-dusting frequently is a must.

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PS, there's never enough RAM in a system. Default when idle/work I've got it on 80/95% respectively of installed. Disk swapping can loose significant time... the HD lights would indicate that.

edit: Added piece on Elapsed/Wallclock time.

cpl of reasonably cheap upgrades, 1. max out the ram 2. if a desktop then add a second PHYSICAL hdd (yes, even an old 20gb drive thats sitting under the workbench unused will do) 3. move the system pagefile/swapfile/virtual memory to the second hdd as well as all temp folders (administrator & user) to the second hdd. Don't forget to reset the pointers for the temp folders in the profiles. As stated in other posts if your cpu is overheating then you will have to reduce the duty cycle of the cpu to keep it from overheating & possibly burning out. Of course if you use a solid state HD as the second HDD then performance will improve even more - I have seen performance improve with the above changes anywhere from 20-30%.

I'd check the "While the processor usage is less than x percent" setting in the Processor Usage tab in the client or "Suspend work if CPU usage is above x % of cpu" in the Device Manager page. Also any date-time restriction that could be set by mistake.