Read the following tips carefully and see if you agree with the authors' opinion, i particularly agree with the second argument when it comes to performance.
According to this site
Everything will depend on your goal.
Improve positioning in search tools
If the focus is to improve results with organic search, a subdomain may not be
the best option, because Google considers the subdomain as a domain
different and not as part of the same site. IE, you will have to
build the authority of your subdomain and this will not generate results
for your main domain. In this case, use a subdirectory as it
will benefit from the authority their domain already possesses, which
facilitates good positioning on search pages.
SERP Domination
If you already have a good position on the first page and your goal is
popularize your business and increase traffic to your website, you should
create a subdomain. You basically open more input ports from
visitors to the content of your site, increasing the chances of
achieve SERP Domination.
SERP Domination is to conquer several links in the first pages of
search engines and not only have a link well positioned. With a
differentiated SEO work for your domain and subdomain, you can
have more links pointing to your site on the first few pages, even
for more specific terms.
For example, Google search results for the keyword
"Inbound Marketing for Traditional Companies" show the domain of
Digital Results and the agency subdomain,
agenciasderesultados.resultadosdigitais.com.br.
According to this other site
The benefits of sub-domains for performance
As we can see when analyzing the anatomy of the domain, when we create a
subdomain we ended up "creating a new website" within our domain
leading.
This offers more organization for your agency and can allocate
improve the different applications of your website and facilitate access to
your customers.
Differentiation by search engines and SEO
Subdomains are seen by search engines as sites other than the main domain.
At first this might look bad for SEO, right?
However, this opens up several opportunities, such as segmentation!
So you can have different segment sites under one
main domain and, with the subdomains, separate their authorities
according to the desired segment. This separation of "authority" for each
subdomain is a good opportunity for agencies working with
blogs or sites from different niches. With this, these agencies can
perform search engine optimization (SEO) work
different for each subdomain and for the main domain.
In practice
Google has the most widely used search engine in the world,
with several applications, such as Google Docs and Google
Adwords. Following good security, performance and
optimization, it also separates your applications using subdomains.
Search Engine: google.com
Google Docs: Docs.google.com
Google Adwords: Adwords.google.com