The review process is somehow short, but I believe they are doing their best to make the research work published as soon as possible, if the content follows the standards.
Also, MDPI journals fall short compared to PLOS but they are respectable and are building their reputation. Charges are proportional to the journal reputation and most times can be waived if the author makes its case of lack of funding and it is completely independent of the review process. See the link: https://academia.stackexchange.com/questions/5466/is-mdpi-a-reputable-academic-publisher