It's the maximum total of your PC processor(s) at normal clock speed. This shouldn't be any cause to your degraded performance issues either. It's normal to read some value in percentage based on how your CPU is clocked speed wise in BIOS, etc.
Maximum Frequency in Resource Monitor is the same as the Processor
Performance% of Maximum Frequency counter in Performance Monitor.
For example if you have a 2.5 ghz processor which is running at 800 mhz
then % of Maximum Frequency = 800/2500 = 31%. So the processor is
running at 31%, or 800 mhz, of the processor's maximum frequency of
2500 mhz (2.5 ghz).
The "best" percentage of maximum frequency is
subjective. Basically, you want the CPU running at a frequency that is
fast enough to do what you want while using the least amount of power
so it doesn't drain your battery or increase your electric bill
unnecessarily. Your power plan in Windows is part of what determines
the frequency as well as settings in the computer's BIOS.
Source