For a great UX, great performance is actually important so that your UI does not block too long on requests that users notice. To get great performance, you need to do things right in your backend. It's not optional. If you do things right, it's going to be easier to scale what you are doing when the time comes. Those two are connected.

Usually where SAAS startups get into trouble is actually the UX and product market fit. There are a lot of successful SAAS companies with absolutely terrible UX. The reason they are successful is that the UX does not matter if what the software does is so valuable that companies want to buy it anyway. So, obsessing about a UX for customers that don't actually care too much may be a mistake. E.g. investing in native apps and tripling the number of frontend engineers you need for that is usually a mistake. Get the software in the hands of your customers quickly and learn whether it lives up to your sales pitch fast. Simple web UI and a simple backend with some conservative tech choices goes a long way. And that stuff is easy to scale.

A lot of SAAS software is bought by people who don't actually use the software themselves. These people buy based on a story and a promise; not the actual UX. There's this curious dynamic where you e.g. sell to the VP of sales who then makes his sales team use the software. The higher the price, the less the UX matters. That's why a lot of SAAS software is absolutely miserable to use: it's designed to make the boss happy, not users. This is also what represents a big opportunity: a lot of that software is not very well defended and can be disrupted with better products. And this happens all the time. But then those companies also end up selling to the same C-level executives and that software becomes less usable over time. Salesforce is the classic example. I've met sales people that hate that stuff with a passion but have to use it anyway. But they started out by disrupting that space.

Ironically, UX matters more if you don't have a remarkable product that is easy to copy. These products depend on easy purchase decisions by people lower in the org chart. Slack is the classic example that showed up in many companies because they had a freemium layer. And then those companies decided to pay up because they had a growing number of people using it.

So, that kind of product actually needs to scale early because it depends on a lot of users not paying anything before enough accounts start converting to paid accounts. Slack only managed that because they had lots of funding. Many companies would have run out of steam long before they could have succeeded.

Those users are going to be a lot more critical on the UX, and it's responsiveness and if the experience sucks, they'll be gone in no time and use something shinier. So, such startups will never break even unless they learn to scale before their runway runs out. For every cute little SAAS app that made it, there are hundreds of me too apps that never came even close to breaking even. Fail fast causes lots of companies to fail. Fast. Unless they can scale and grow. This is why investors are weary of B2C as well. Same scaling issues and it's a lot harder to get consumers to pay. And the UX is critical.

More traditional SAAS companies can afford to close a few big deals with a mediocre product before they have to scale. Mostly these companies keep on selling to bigger and bigger companies without ever needing to address their UX issues.