I’m not sure what causes this to happen but if you change your logical files to rebuild the access paths during your IPL, you will not suffer the performance hit you describe. However, your IPL may take longer.
Here is an article that give and idea of what to do.
That sounds like what may have happened… After a power loss here ( before our UPS ) we had the same type of issue. On the IPL we bypassed the re-building of the LF’s and response time after the IPL was done was painfully slow.
The first time a logical file is accessed it rebuilds the index when the file opens.
A recompile will build it as part of the process by default.