On the face of it, your application was designed to cope with a given volume and your business is now exceeding the design volume of the application.

My advice is that your management ask for a report on the scalability of your application, and ask it to assess the infrastructure needed if the business doubled in size over the next 3 years. This should identify the pain points you will encounter, and lead to a strategy to resolve them. If you keep with your custom app you may be faced with a large re-write process - which as far as adding value to the busines is concerned is just running up cost only to stand still.

One short-term option is to look at sharding. Typically this involves looking at a DBMS that is designed to be sharded, but any move like this is likely to cost similar to a total rewrite. An alternative is to shard your customer base.

If you work out what size of customer base your application can cope with and still be reliable and cheap to run, you divide this into your current or projected customer base and get the number of instances of the application you need. You will then need to write some new front-end that can get key customer information and direct the rest of the work to the appropriate instance of your application.

Whatever option you choose thre will be complexity, risk and cost. However, if you as a techie are seeing that the application is getting near the limits of its scalability, then you need to tell management and they need to get some work commissioned to find out the true extent of the problem.

If your business does not address the underlying scalability issues then you are into sticking band-aids over the wounds. In the same way that a bank is now just an IT shop that also lends money and a supermarket is an IT shop that also distributes food, your business relies and totally depends on working IT in order to survive. There is a history of failed companies that relied on sweating the assets a bit too far rather than upgrading the technology.