Understanding SQL Server Cache
but at night it is deleted because of the maintenance task


I would not say deleted as more "overwritten" or "evicted". Maintenance may simply use the memory for other things. Not a big issue normally.


  and so a next day, when customers do transactions, data pages have to be again loaded 
  to  Cache, which takes time and slows a lot of things down.


If that is really that brutal the case, you have either an edge case or a hardware problem.

Generally you can preseed the cache by running a number of "maintenance queries" in the early morning (to reload the data into the cache at 0600 in the morning for example) as part of maintenance. I.e. you end it by querying for the data you want loaded into the cache. Problem solved.

But generally this should not result in an EXTREME slowdown - yes, initial requests will be slower, but that should fix itself quite fast as the cache is going to fill up with every query. Is it possible that your experienced slowdown is not "cache" but "morning spike"? There are a lot of systems where at specific times (one of them the morning when people log in and check their work) the load is a lot higher than later during the day. In this case, you need to scale the hardware up to handle this. WOuld be among the first times I would have seen maintenance as a very significant issue.