According to his numbers, the processor was restricted to a maximum speed of 2.2GHz, far below the listed of 2.9GHz.
Processing speed enables computers to take on multiple tasks at once.
In his review, Dave Lee used Adobe Premiere, a piece of video-editing software, to test the machine's capacity.
He found the model without the additional upgrade to the i9 chipset was still capable of hitting the same 2.6GHz speeds – the same as the throttled processor.
"The i9 in the MacBook can't even maintain the base clock speed," Lee said.
"Forget about Turbo Boost, it can't even maintain the 2.9GHz base clock speed, which is absurd.
"This CPU is an unlocked, overclock-able chip, but all of that CPU potential is wasted inside this chassis, and the thermal solution inside this chassis."
The limitations of the view could be misleading, with Lee only testing a single task in a single app.
It is also important to acknowledge the test was only done on a single computer, and could reflect a flaw with this particular computer and not on the model as a whole.
Apple declined to comment on the claims.
This is not the first time Apple has experienced issues with the reliability and performance of its laptops.
The so-called "butterfly keyboard" has received a flood of complaints after MacBook owners claimed keys would repeat unexpectedly, respond inconsistently, or stop working entirely on their notebooks.
Last month, Apple responded by claiming it would fix some models for free.
The design of the keyboard means MacBook owners are unable to remove debris trapped underneath the keys at home.
As a result, Apple has announced it will be cover the costs of any keyboard replacements, and reimbursing those who have already footed the bill themselves.
The new keyboard service programme covers nine notebooks launched since 2015.
The Cupertino-based company has already been hit with three class-action lawsuits around the super-thin keyboards.
Apple first introduced the new keyboard design in 2015, with the launch of its 12-inch MacBook.
To accommodate a full-size keyboard into the small frame of the notebook, which measures just 0.35 cm (0.14 inch) at its thinnest point, Apple reduced the travel of the keys with its new "butterfly-mechanism".
