It depends on the phase your business is in. For the beginning using standardized stuff that you buy ready made can be a real time saver, and time is usually in short supply. Once you achieve a certain scale and you can afford to do it you can usually save substantially and scale up further by doing something more customized.
Starting off with a complete custom set-up for your core is another opportunity for premature optimization to creep in.
I've seen this happen where a mature business starts to functionally decomposes their business down to what they do and how they deliver. They then go out and buy products that do those things and link them in a chain with a DB. But each one of those is short about 10-20% in all the used to haves and nice to haves.
So what then dawns on the business is they realize that the missing 10-20% was part of the business that was really important and they have dropped serious money on a bunch of products. And really all they needed to do was better understand themselves and build their own business infrastructure.
But what you are saying about speed definitely rings true. But it's important to note IT failures that happen to new businesses are more or less written off as total business failures Usually resulting in the business going to the wall.
Starting off with a complete custom set-up for your core is another opportunity for premature optimization to creep in.