At the risk of being too general/closed for off-topic, before you choose any platform, I would ask these questions:
- What sort of functionality does your client require—now or in the future?
- What kinds of products will the store support (eg bundles, custom products, variants, donations, etc).
- What sort of budget / developer support can the client support or is required?
In general, Craft is the more nascent/new product so if your team's dev chops aren't familiar with Commerce, there can be a learning curve.
Magento does a lot of the box and if you need to hook into some obscure (or not so obscure) service, Magento probably already has a plugin for it. It has a bigger ecosystem but that doesn't mean Craft isn't up to the task.
Shopify can also be an option depending on what you're trying to do.
In general, Craft/Commerce really excels at customized e-commerce. If your client's needs are modest, for example selling simple products without customization, Shopify might actually be a better fit long term. However when you start to get into more niche businesses where there's customized shipping or products, the the tables tip more in favor of Craft.
You question also asks about speed.
Fast is a relative turn. A lot is going to depend on 1) server infrastructure and 2) your own code.
- What does a performance budget look like?
- How many visitors/users are you expecting? Where are they located?
- How will you measure the solution?
Compared to Magento, Craft is pretty light-weight. Magento is not known for being resource-friendly, however there is probably more documentation on how to make it speedy.
If you go with something like Shopify, you don't have to worry about infrastructure—which is one of the reasons it's popular.
Why choose Craft Commerce over Magento