AWS Lambda performance initializing takes long due to required file (XSSFWorkbook excel instance) upload time

Question

AWS Lambda initialization issue:

We got an app (lambda based architecture) that uses an excel Spreadsheet (10MB size) which is loaded using apache POI XSSFWorkbook instance. Here what we have tried:

1. First we tried to load the spreadsheet at the init phase, but this was problematic since the startup of the lambda was taking long and generating the lambda to restart multiple times to finally get it up. (This is due to the init time limit of lambdas of 10 seconds. See this). Some times the response was just a timeout error (the api gateway limit).

2. We moved the load of the spreadsheet to the function so this was not longer part of the init phase. (Once the instance is created, that's the instance to be used while the lambda is up). This solved the time out error and the init restart. But the lambda takes close to 15 seconds (which is not acceptable) to return the response in the first call, then it behaves normally.

3. We tried provisioned concurrency, which allows to have lambdas 'warm' but this feature depends on the requests, so it works fine when lambda is frequently demanded, but when it is not, (two requests within 20 minutes or less), the lambda will need to go through the whole spreadsheet upload process (only init phase will keep warm) meaning, we will get again a 16 seconds request.

Is there any advice or any possible solution you can think of to manage this situation in order to never get this >16 secs request? (get the spreadsheet available in memory even if the lambda has not received requests).

Thanks in advance.